merge revision(s) r48111,r48118,r48120: [Backport #10421]

	* vm_method.c (rb_method_entry_make): warn redefinition only for
	  already defined methods, but not for undefined methods.
	  [ruby-dev:48691] [Bug #10421]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@48138 b2dd03c8-39d4-4d8f-98ff-823fe69b080e