Stopped sending empty frame on stream initiated in sender half-closed state.

The SPDY protocol states the sender of FLAG_FIN must not send further frames
on that stream.  This change skips the sending of a final (empty) frame with
FLAG_FIN set when the stream is already half-closed because SYN_STREAM was
sent with FLAG_FIN set.
Merge pull request #246 from kriswuollett/no_fin_on_closed_stream

Stopped sending final empty frame on stream initiated in sender half-closed state.