host: Avoid zero-length attach frame writes
host: Avoid zero-length attach frame writes

If the entire message is just a newline, a zero-length frame was written
which indicates a stream EOF to the client. Solved by including the
trailing newline in the same write.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

flynn/flynn,benbjohnson/flynn,benbjohnson/flynn,philiplb/flynn,philiplb/flynn,benbjohnson/flynn,flynn/flynn,flynn/flynn,philiplb/flynn,benbjohnson/flynn,philiplb/flynn,lmars/flynn,benbjohnson/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,benbjohnson/flynn,lmars/flynn,lmars/flynn