This seems to be working on JRuby 9K
This seems to be working on JRuby 9K

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
jruby 1.7.26 (1.9.3p551) 2016-08-26 69763b8 on Java HotSpot(TM) 64-Bit Server VM 1.8.0_45-b14 +jit [darwin-x86_64]
"\u307B\u3052"

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
jruby 9.1.7.0 (2.3.1) 2017-01-11 68056ae Java HotSpot(TM) 64-Bit Server VM 25.45-b02 on 1.8.0_45-b14 +jit [darwin-x86_64]
"\u307B\u3051\u3099"

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin14]
"\u307B\u3051\u3099"

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