wrap Metadata more correctly

to conform with `jsonobject`'s greater strictness.

`couchdbkit` was totally cool with you doing something like

`Metadata({'timeStart': datetime.datetime(2013, 1, 1, 12, 0, 0)})`,

whereas that's really incorrect since you should be wrapping pure json:

`Metadata({'timeStart': '2013-01-01T12:00:00')})`.

Moreover, in this particular case, `self._form` is a "wrapped" object, whereas the constructor expects a "not-wrapped" object. (That's mainly a conceptual difference, though self._form is actually a different type, `couchdbkit.LazyDict`.)

Using the ** notation is essentially the same as `Metadata(timeStart=datetime.datetime(2013, 1, 1, 12, 0, 0))` which is legit (and `Metadata(timeStart='2013-01-01T12:00:00'))` is NOT legit).