Support infinity in PostgreSQL date columns

This adds support for the special input string values `infinity` and `-infinity` stored in PostgreSQL date columns.

When reading from the database the PostgreSQL adapter now returns `Float::INFINITY` when the stored value equals `infinity` and `-Float::INFINITY` when it equals `-infinity`.

Until now Rails only had support for Infinity in `float` and `datetime` columns. We need this since we have start and end dates in our project. The end date can be infinity.

Please let me know if this change is okay or if I should add something. I tried to use the same code as in `OID::DateTime`.
in my understanding the `case` statement should work with none-strings as well, do you really need the if statement?