erb.rb: Allow explicit trimming carriage return

	when trim_mode is "-", for Windows environments.

	[ruby-core:39625] [Bug #5339]
	erb.rb: Allow trimming CR in all trim_modes

	to unify a behavior with r58823 and r58825.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59301 b2dd03c8-39d4-4d8f-98ff-823fe69b080e