Only encode headers that have non-ascii characters.

So, we have to be even clever-er to avoid running certain headers
(namely the MD5 checksup) through urllib.quote_plus.

Honestly at this point we should probably just check for it explicitly.
Remove a hack.

So in 2.5 and below, apparently "host" had to be amended to "host:port"
for compatibility with httplib.  The code would check to see if we
were running 2.6 and 2.7, which in 3.3 we *aren't*, but I guess it
never got the memo.

2.5 and older aren't supported now anyway, so remove the check entirely.