Another Ruby 2.6 BigDecimal compatibility issue
Another Ruby 2.6 BigDecimal compatibility issue

This patch modifies XmlMini::Parsing["decimal"] to handle a string that
contains an invalid number. Since [ruby/ruby@a0e438c#diff-6b866d482baf2bdfd8433893fb1f6d36R144](https://github.com/ruby/ruby/commit/a0e438cd3c28d2eaf4efa18243d5b6edafa14d88#diff-6b866d482baf2bdfd8433893fb1f6d36R144) this case raises an `ArgumentError`. `String.to_f` returns 0.0 if there is not a valid number at the start of the argument, so current behavior is conserved.

See https://travis-ci.org/rails/rails/jobs/463180341#L6264

Related: #34600, #34601

georgeclaghorn/rails,eileencodes/rails,notapatch/rails,joonyou/rails,Erol/rails,shioyama/rails,prathamesh-sonpatki/rails,Edouard-chin/rails,bogdanvlviv/rails,repinel/rails,mechanicles/rails,yalab/rails,lcreid/rails,joonyou/rails,flanger001/rails,georgeclaghorn/rails,schuetzm/rails,rails/rails,starknx/rails,esparta/rails,rafaelfranca/omg-rails,esparta/rails,rails/rails,repinel/rails,jeremy/rails,iainbeeston/rails,yhirano55/rails,iainbeeston/rails,kmcphillips/rails,kddeisz/rails,Envek/rails,repinel/rails,kmcphillips/rails,tgxworld/rails,joonyou/rails,Stellenticket/rails,betesh/rails,shioyama/rails,kmcphillips/rails,georgeclaghorn/rails,vipulnsward/rails,fabianoleittes/rails,betesh/rails,Erol/rails,utilum/rails,jeremy/rails,yhirano55/rails,MSP-Greg/rails,palkan/rails,yalab/rails,iainbeeston/rails,schuetzm/rails,fabianoleittes/rails,Vasfed/rails,Envek/rails,notapatch/rails,kmcphillips/rails,betesh/rails,utilum/rails,MSP-Greg/rails,Stellenticket/rails,mechanicles/rails,shioyama/rails,tjschuck/rails,bogdanvlviv/rails,EmmaB/rails-1,Vasfed/rails,yahonda/rails,prathamesh-sonpatki/rails,rails/rails,tgxworld/rails,Edouard-chin/rails,lcreid/rails,rafaelfranca/omg-rails,yhirano55/rails,jeremy/rails,palkan/rails,tjschuck/rails,Vasfed/rails,shioyama/rails,repinel/rails,lcreid/rails,arunagw/rails,kddeisz/rails,Edouard-chin/rails,fabianoleittes/rails,bogdanvlviv/rails,prathamesh-sonpatki/rails,utilum/rails,eileencodes/rails,notapatch/rails,yalab/rails,BlakeWilliams/rails,esparta/rails,palkan/rails,Erol/rails,eileencodes/rails,kddeisz/rails,Erol/rails,palkan/rails,BlakeWilliams/rails,flanger001/rails,tgxworld/rails,yhirano55/rails,yawboakye/rails,BlakeWilliams/rails,yahonda/rails,EmmaB/rails-1,notapatch/rails,Stellenticket/rails,gauravtiwari/rails,tjschuck/rails,Vasfed/rails,schuetzm/rails,lcreid/rails,vipulnsward/rails,tgxworld/rails,iainbeeston/rails,rafaelfranca/omg-rails,Envek/rails,jeremy/rails,gauravtiwari/rails,yahonda/rails,yahonda/rails,Edouard-chin/rails,arunagw/rails,prathamesh-sonpatki/rails,tjschuck/rails,yawboakye/rails,arunagw/rails,betesh/rails,starknx/rails,BlakeWilliams/rails,EmmaB/rails-1,mechanicles/rails,flanger001/rails,flanger001/rails,gauravtiwari/rails,starknx/rails,bogdanvlviv/rails,georgeclaghorn/rails,rails/rails,utilum/rails,yawboakye/rails,yalab/rails,vipulnsward/rails,arunagw/rails,MSP-Greg/rails,esparta/rails,eileencodes/rails,Stellenticket/rails,MSP-Greg/rails,yawboakye/rails,mechanicles/rails,joonyou/rails,schuetzm/rails,vipulnsward/rails,fabianoleittes/rails,Envek/rails,kddeisz/rails