proc.c: skip prepended modules

	* proc.c (method_super_method): skip prepended modules and
	  continue from the super class of the original class.
	  [ruby-core:81666] [Bug #13656]
	test/ruby/test_method.rb: refined  [ruby-core:81666] [Bug #13656]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@59231 b2dd03c8-39d4-4d8f-98ff-823fe69b080e