Fix to_yaml test when run individually
Fix to_yaml test when run individually

The to_yaml method is undefined when running the test as:

  $ ruby -I lib:test test/core_ext/string_ext_test.rb

Doesn't fail when running rake test:isolated presumably because
something else has required 'yaml' already.

starknx/rails,brchristian/rails,schuetzm/rails,Erol/rails,Edouard-chin/rails,joonyou/rails,flanger001/rails,lcreid/rails,tgxworld/rails,mathieujobin/reduced-rails-for-travis,rails/rails,esparta/rails,bogdanvlviv/rails,ngpestelos/rails,illacceptanything/illacceptanything,esparta/rails,vipulnsward/rails,kddeisz/rails,BlakeWilliams/rails,coreyward/rails,shioyama/rails,hanystudy/rails,stefanmb/rails,mtsmfm/railstest,Stellenticket/rails,joonyou/rails,coreyward/rails,tijwelch/rails,assain/rails,kaspth/rails,elfassy/rails,untidy-hair/rails,mtsmfm/rails,odedniv/rails,rafaelfranca/omg-rails,koic/rails,illacceptanything/illacceptanything,deraru/rails,MichaelSp/rails,vipulnsward/rails,kamipo/rails,felipecvo/rails,printercu/rails,maicher/rails,Envek/rails,baerjam/rails,assain/rails,notapatch/rails,prathamesh-sonpatki/rails,BlakeWilliams/rails,ledestin/rails,Stellenticket/rails,sergey-alekseev/rails,gfvcastro/rails,georgeclaghorn/rails,ttanimichi/rails,Vasfed/rails,rails/rails,baerjam/rails,arunagw/rails,voray/rails,alecspopa/rails,xlymian/rails,arunagw/rails,kaspth/rails,jeremy/rails,illacceptanything/illacceptanything,maicher/rails,utilum/rails,deraru/rails,mtsmfm/rails,illacceptanything/illacceptanything,gauravtiwari/rails,betesh/rails,rbhitchcock/rails,felipecvo/rails,arjes/rails,eileencodes/rails,ttanimichi/rails,alecspopa/rails,printercu/rails,bradleypriest/rails,Vasfed/rails,MichaelSp/rails,iainbeeston/rails,georgeclaghorn/rails,Vasfed/rails,MSP-Greg/rails,gcourtemanche/rails,travisofthenorth/rails,yasslab/railsguides.jp,kenta-s/rails,mohitnatoo/rails,Erol/rails,sergey-alekseev/rails,gfvcastro/rails,fabianoleittes/rails,betesh/rails,tjschuck/rails,deraru/rails,bogdanvlviv/rails,kaspth/rails,mohitnatoo/rails,baerjam/rails,repinel/rails,yawboakye/rails,iainbeeston/rails,illacceptanything/illacceptanything,sealocal/rails,travisofthenorth/rails,ledestin/rails,rossta/rails,fabianoleittes/rails,printercu/rails,kirs/rails-1,matrinox/rails,gavingmiller/rails,shioyama/rails,MichaelSp/rails,mechanicles/rails,printercu/rails,arunagw/rails,deraru/rails,kenta-s/rails,notapatch/rails,yasslab/railsguides.jp,yhirano55/rails,kddeisz/rails,brchristian/rails,rails/rails,repinel/rails,ledestin/rails,iainbeeston/rails,jeremy/rails,gfvcastro/rails,Envek/rails,yhirano55/rails,stefanmb/rails,marklocklear/rails,yalab/rails,MSP-Greg/rails,notapatch/rails,odedniv/rails,sealocal/rails,matrinox/rails,ngpestelos/rails,yasslab/railsguides.jp,pschambacher/rails,vipulnsward/rails,mtsmfm/railstest,georgeclaghorn/rails,illacceptanything/illacceptanything,tijwelch/rails,hanystudy/rails,rafaelfranca/omg-rails,flanger001/rails,gcourtemanche/rails,repinel/rails,joonyou/rails,lucasmazza/rails,illacceptanything/illacceptanything,palkan/rails,ttanimichi/rails,pvalena/rails,Edouard-chin/rails,riseshia/railsguides.kr,betesh/rails,odedniv/rails,mohitnatoo/rails,richseviora/rails,schuetzm/rails,koic/rails,utilum/rails,pvalena/rails,Erol/rails,bolek/rails,Spin42/rails,Edouard-chin/rails,betesh/rails,lucasmazza/rails,xlymian/rails,sergey-alekseev/rails,Sen-Zhang/rails,tgxworld/rails,arjes/rails,Envek/rails,kddeisz/rails,tjschuck/rails,pvalena/rails,utilum/rails,georgeclaghorn/rails,pvalena/rails,mtsmfm/railstest,kmcphillips/rails,shioyama/rails,marklocklear/rails,gauravtiwari/rails,kachick/rails,kmcphillips/rails,prathamesh-sonpatki/rails,untidy-hair/rails,alecspopa/rails,hanystudy/rails,flanger001/rails,EmmaB/rails-1,samphilipd/rails,eileencodes/rails,rossta/rails,bradleypriest/rails,aditya-kapoor/rails,elfassy/rails,yawboakye/rails,kamipo/rails,jeremy/rails,bogdanvlviv/rails,rbhitchcock/rails,riseshia/railsguides.kr,kmayer/rails,mtsmfm/rails,illacceptanything/illacceptanything,palkan/rails,yahonda/rails,illacceptanything/illacceptanything,mijoharas/rails,gavingmiller/rails,starknx/rails,Spin42/rails,pschambacher/rails,kirs/rails-1,bogdanvlviv/rails,xlymian/rails,richseviora/rails,yhirano55/rails,esparta/rails,rails/rails,flanger001/rails,mohitnatoo/rails,bolek/rails,pschambacher/rails,bradleypriest/rails,Stellenticket/rails,MSP-Greg/rails,untidy-hair/rails,kmcphillips/rails,rafaelfranca/omg-rails,maicher/rails,richseviora/rails,mechanicles/rails,jeremy/rails,joonyou/rails,kachick/rails,riseshia/railsguides.kr,gcourtemanche/rails,kenta-s/rails,koic/rails,brchristian/rails,samphilipd/rails,yalab/rails,bolek/rails,mathieujobin/reduced-rails-for-travis,mechanicles/rails,arjes/rails,travisofthenorth/rails,eileencodes/rails,travisofthenorth/rails,starknx/rails,untidy-hair/rails,matrinox/rails,aditya-kapoor/rails,shioyama/rails,Edouard-chin/rails,Spin42/rails,fabianoleittes/rails,yahonda/rails,coreyward/rails,illacceptanything/illacceptanything,lcreid/rails,palkan/rails,gfvcastro/rails,felipecvo/rails,fabianoleittes/rails,arunagw/rails,samphilipd/rails,schuetzm/rails,kmayer/rails,BlakeWilliams/rails,baerjam/rails,lcreid/rails,aditya-kapoor/rails,elfassy/rails,aditya-kapoor/rails,utilum/rails,voray/rails,rossta/rails,kmayer/rails,kirs/rails-1,vipulnsward/rails,esparta/rails,illacceptanything/illacceptanything,lucasmazza/rails,sealocal/rails,Sen-Zhang/rails,yawboakye/rails,kmcphillips/rails,assain/rails,yawboakye/rails,marklocklear/rails,yalab/rails,prathamesh-sonpatki/rails,yalab/rails,mijoharas/rails,mechanicles/rails,voray/rails,mijoharas/rails,Erol/rails,illacceptanything/illacceptanything,yahonda/rails,tgxworld/rails,kamipo/rails,illacceptanything/illacceptanything,MSP-Greg/rails,tgxworld/rails,yahonda/rails,Vasfed/rails,tjschuck/rails,stefanmb/rails,palkan/rails,gauravtiwari/rails,repinel/rails,illacceptanything/illacceptanything,assain/rails,eileencodes/rails,Stellenticket/rails,iainbeeston/rails,Envek/rails,gavingmiller/rails,tjschuck/rails,kddeisz/rails,riseshia/railsguides.kr,rbhitchcock/rails,illacceptanything/illacceptanything,Sen-Zhang/rails,EmmaB/rails-1,kachick/rails,yasslab/railsguides.jp,tijwelch/rails,EmmaB/rails-1,mathieujobin/reduced-rails-for-travis,prathamesh-sonpatki/rails,ngpestelos/rails,notapatch/rails,schuetzm/rails,BlakeWilliams/rails,yhirano55/rails,lcreid/rails