KeyEntity and Identity items should not use kSecReturnData

[macblazer]

According to Apple's documentation for returning keychain items:

"""
Don’t use kSecReturnData for cryptographic key or identity items, as the key material may not be extractable. Instead, call SecKeyCopyExternalRepresentation(::), and check the error parameter if it returns nil.
"""

Probably we can hide this implementation detail within Haversack when running a KeyQuery that has kSecReturnData in it.

Be sure to test with keys that are not extractable.