Improve specs for Time#gmtime/Time#utc

[herwinw]

The Time comparison does cancel out the timezone differences, so even if you did not call the method, the spec would still pass.

[herwinw]

Todo: validate/extend checks for Time#== (or Time#<=>)

[herwinw]

The second commit adds some extra checks for Time#<=>. I picked CET and BRT pretty much at random for the time zones other than UTC, I wanted something before UTC and something after UTC, and these just happened to be the ones I knew from memory.