This commit fixes issue #157

With this commit:
1) SQLite tests are not affected
2) MongoDB tests are not affected, and UTCTime is tested if high_precision_flag is specified
3) PostgreSQL date tests pass (with the exception of ZonedTime, which will never pass, see link in code)
4) MySQL/MariaDB date tests will pass if previosly mentioned [issue](https://github.com/bos/mysql-simple/issues/6) is solved (but they were not passing before this patch either).

Regarding 4), if the issue is not solved, the approach taken to convert to microseconds can be replicated to convert to seconds.

PostgreSQL tests are still one test away from passing, due to a 'double precision' issue that I will create next.