proc.c: fix method proc binding receiver

* proc.c (method_proc): the receiver of binding from method should
  be same as the receiver of the method.
  [ruby-core:65917] [Bug #10432]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48160 b2dd03c8-39d4-4d8f-98ff-823fe69b080e