Only call Socket#write once per message send

We were calling #write twice; once to send the header, again to send the
body. While this works, it sometimes causes network fragmentation which
leads to a nasty IncompleteRexProRequestException warning from rexster
and introduces extra latency in the request.

Concatenate the header and body and write all at once instead.

Fixed unrelated line length issue in spec.

Fixes #3