Fix dynamic datetimes

Changelog:

 * When a DateTime or DateTimeOffset is serialized as a ```Edm.String``` instead of a ```Edm.DateTime```, the string ```%HYDE_{type}%``` is prepended to the field. This is to allow hyde to correctly assign a type when creating a dynamic object from the table storage data.

Given that this is a different data storage format than the 5.3.1 update, this is a **breaking** change. However, given that it's only been live for a day or two, I'm not sure that anyone has used it yet.

Second thought:
We could explore an approach where our dynamic deserializer indiscriminately tries to parse a string as a ```DateTime``` and returns it as one if it can. This would prevent this from being a breaking change, but might be weird behavior.  (Maybe I deliberately serialized a DateTime as a string).