encoding.c: find encoding index
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51670 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
encoding.c: find encoding index
making a string object every time. [ruby-core:58160] [Bug #9080]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51670 b2dd03c8-39d4-4d8f-98ff-823fe69b080e