bug_10591 - looking deeper, RFC2616 seems to imply an empty status, so I support that, with and without a trailing space as also implied by the RFC

Reason-Phrase = *<TEXT, excluding CR, LF>

Because of the star (*) I read this as "empty reason phrase is allowed".
(http://grokbase.com/t/apache/dev/085mz81ecq/empty-reason-phrase-bz-44995-45092)