Assign blessed_key to metadata if blessed_key is defined

[wbarnha]

Fixes #421.

[wbarnha]

Apparently options.allow_blessed_key is defined in faust/types/models.py, do we want to add this as well to check before adding the blessed key?

[wbarnha]

faust/faust/models/base.py

Lines 153 to 160 in ebc7777

	#: Serialized data may contain a "blessed key" that mandates
	#: how the data should be deserialized. This probably only
	#: applies to records, but we need to support it at Model level.
	#: The blessed key has a dictionary value with a ``ns`` key:
	#: data = {.., '__faust': {'ns': 'examples.simple.Withdrawal'}}
	#: When ``Model._maybe_reconstruct` sees this key it will look
	#: up that namespace in the :data:`registry`, and if it exists
	#: select it as the target model to use for serialization.

Guess we don't need the option here.