send-cmd should prepend last arg with : only if /\s/

[Summertime]

edit: might be best to squash considering the self-bikeshedding I've done with this single line

ideally according to the IRC specifications, : is optional if the last argument to a command has no whitespace

This is to help alleviate some difficulties in zoffixznet/perl6-banbot#2

I am not completely sure all servers agree with the IRC specification on : however! Hopefully this is safe enough change to make

[Summertime]

oh, there is a mistake, last arg beginning with : will need : prepended

[zoffixznet]

:+1: thanks