merge revision(s) 35112,35121: [Backport #6190]

	* transcode.c (str_encode_bang, encoded_dup): if nothing was
	  transcoded, just set encoding but leave coderange unchanged as
	  forcee_encoding.  [ruby-core:43557][Bug #6190]

	* transcode.c (documentation for str_encode): Explain
	  that transcoding to the same encoding is a no-op
	  (i.e. no exceptions, no replacements,...).
	  [ruby-core:43557][Bug #6190]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@40056 b2dd03c8-39d4-4d8f-98ff-823fe69b080e