Use correct syntax for Last-Modified header (#172)

See 'https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Last-Modified#syntax'

lib/phlex/rails/streaming.rb

@@ -10,7 +10,7 @@ def stream(view)
 		headers["X-Accel-Buffering"] = "no"
 		headers["Cache-Control"] = "no-cache"
 		headers["Content-Type"] = "text/html; charset=utf-8"
-		headers["Last-Modified"] = Time.zone.now.ctime.to_s
+		headers["Last-Modified"] = Time.now.httpdate
 
 		response.status = 200