Fixes bug disallowing custom-made reason phrase

The send_error method is, I believe, supposed to allow sending an HTTP reason phrase that is not one of the standard ones. This change corrects an apparent bug which made this impossible. With the correction, specifying an 'error' keyword argument to send_error actually sends the specified
reason phrase to the client.
Merge pull request #1102 from pekrau/master

Add 'reason' kwarg to send_error.