proc.c: skip prepends

* proc.c (mnew): skip prepending modules and return the method bound
  on the given class.  [ruby-core:52160] [Bug #7836]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39224 b2dd03c8-39d4-4d8f-98ff-823fe69b080e