Pinboard.tags.get() breaks if tag key is "date" as it matches DATE_FIELDS

[mpslanker]

This is probably very similar to issue #19.

However, just as it says in the title. If you have tag keys that match on any of the field lists, it breaks.
For example, if you have a tag of "date" then at line 199 the tag count is passed to Pinboard.datetime_from_string().
This results in an error as a string, in the form of a date, was expect and not an int.