handling signed messages?

Hi,

Thanks for the awesome library. This is not a bug report so much as an assistance request.

When I sign a message as per the doc, the "signed" result is a string, both:

isinstance(signed, str)

and

isinstance(signed, nacl.signing.SignedMessage)

return true.

However I can't seem to encode the signed message like I can with all of the other provided objects.

When I attempt:

In [1]: import nacl.utils

In [2]: import nacl.signing

In [3]: signing_key = nacl.signing.SigningKey.generate()

In [4]: signed = signing_key.sign(b"abc123")

In [5]: signed.encode(encoder = nacl.encoding.HexEncoder)

I get the following error:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-c597ca2a169e> in <module>()
----> 1 signed.encode(encoder = nacl.encoding.HexEncoder)

TypeError: 'encoder' is an invalid keyword argument for this function

So I used base64.b64encode to encode the signed message for transport, however once decoded on the other side I no longer have access to the "signed.signature" and "signed.message" attributes.

In [11]: signed.message
Out[11]: 'abc123'

In [12]: encoded = base64.b64encode(signed)

In [13]: decoded = base64.b64decode(encoded)

In [14]: encoded.message
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-14-68a604c6014b> in <module>()
----> 1 encoded.message

AttributeError: 'str' object has no attribute 'message'

Do I have to parse the signed message myself?
Is there any way to cast the signed str back to nacl.signing.SignedMessage to programmatically retrieve signed.message?

Start by reading the signing documentation and the nacl.signing.SignedMessage behavior described in the report. Determine whether the intended outcome is documentation guidance or a supported encode/decode API, then define the expected round-trip behavior and document or test it accordingly.