Fixed 32-bit integer overflow by allowing UTCDateTime() to get it's own current time

[macdabby]

@jenssegers This fixes the issue over on #1115

[macdabby]

I ran tests with this code in the freshTimestamp() method:

    print new UTCDateTime().get;
    print "\n";
    print new UTCDateTime((int)round(microtime(true) * 1000));
    print "\n";

What's interesting is that the output was as follows:

1487777270195
1487777270195
1487777270197
1487777270197
1487777270197
1487777270197
1487777270197
1487777270198
1487777270197
1487777270198
1487777270198
1487777270198
1487777270198
1487777270198
1487777270198
1487777270199
1487777270198
1487777270199

In most cases, you'll see the matching pair, however in a few cases they were off by one second. I suppose that comes from rounding.

[jenssegers]

Can you change src/Jenssegers/Mongodb/Auth/DatabaseTokenRepository.php as well?

[macdabby]

Yeah, I also added an explicit null as a parameter to prevent notices. Tested to make sure I got the same output as above.