get rid of test error/failure on Windows introduced at r62955

	* lib/webrick/httpresponse.rb (send_body_io): use seek if NotImplementedError
	  is raised in IO.copy_stream with offset.

	* lib/webrick/httpservlet/filehandler.rb (multipart_body): ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@63011 b2dd03c8-39d4-4d8f-98ff-823fe69b080e