Cannot deserialise a str payload with mgspec.Raw #317

Description

When deserialising a payload and specifying one of the fields of type msgspec.Raw, it prohibits passing payload as str.

Code to reproduce:

import msgspec

class Foo(msgspec.Struct):
    d: msgspec.Raw

data = '{"d": {"test": "A"}}'

decoder = msgspec.json.Decoder(Foo)
decoder.decode(data)

Output:

# python3 --version
Python 3.10.6

# python3 -m pip freeze | grep msgspec
msgspec==0.13.1

# python3 foo.py
Traceback (most recent call last):
  File "./foo.py", line 10, in <module>
    decoder.decode(data)
TypeError: a bytes-like object is required, not 'str'

Changing the payload to a bytes works as expected. Also worth noting that removing Raw and changing it to a proper type works also as expected.

I would like to have a go at fixing this bug, but don't know where to start, as there is no code that points to Raw not allowing a string (which if you pass a value directly to it, it works as expected):

>>> import msgspec
>>> msgspec.Raw("test")
<msgspec.Raw object at 0x7c3889bb70>

so will have to look further into it. Any help in the right direction is appreciated :)