Don't send trailing space in MODE messages

Under some circumstances ngIRCd currently issues a channel MODE message
with a trailing space after the last parameter, which isn't permitted by
the grammar in RFC 2812 section 2.3.1:

http://tools.ietf.org/html/rfc2812#section-2.3.1

The following patch modifies mode-test.e to expose this, and modifies
irc-mode.c to correct it.