webrick: favor .write over << method

	This will make the next change to use IO.copy_stream
	easier-to-read.  When we can drop Ruby 2.4 support in a few
	years, this will allow us to use writev(2) with multiple
	arguments for headers and chunked responses.

	* lib/webrick/cgi.rb (write): new wrapper method
	  lib/webrick/httpresponse.rb: (send_header): use socket.write
	  (send_body_io): ditto
	  (send_body_string): ditto
	  (send_body_proc): ditto
	  (_write_data): ditto
	  (ChunkedWrapper#write): ditto
	  (_send_file): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62988 b2dd03c8-39d4-4d8f-98ff-823fe69b080e