Fixes #4083, make the split nil-safe

In the reported case, the expected cookies were not present on the
response, thus, the second split was trying to split a `nil`. This
solves the immediately problem by a) splitting up the splits into
discrete sections, and b) `NilClass#to_s`'ing the result of the first
split.

This makes the split safe. Now, there may be a larger issue here where
you're not getting the expected cookies -- it sounds like the target in
this case is responding differently, which implies that the module isn't
going to be effective against that particular target. But, at least it
won't crash. It may merely try fruitlessly the entire run, though. I
can't know without looking at a pcap, and in the reported case, a pcap
seems unlikely since this was a bug found in the field.
Ensure necessary cookies are present in OWA 2010 login response
Set proper Cookie header from built cookie string
Show more helpful vprint messages when login fails
Abort if 2013 desired but redirect didn't happen
Prevent calling match when there is no WWW-auth header
Incorporate @jhart-r7's better fix
Updates to better OWA fix
Move userpass emptiness checking into setup
Move redirect check before body check -- a redirect won't have a body
Update description to remove comments about defaults.  Default to 2013
Make OWA 2013 the default, other fixes

Thanks @jhart-r7!

See rapid7#4083 and see rapid7#4094
Land #4094, fixes for OWA brute forcer

Fixes #4083

Thanks TONS to @jhart-r7 for doing most of the work on this!