Fix “object supporting the buffer API required” for multipart requests

[Envek]

This fixes https://github.com/aidan-/httpie-aws-authv4/issues/10 (see this original issue for more details)

The problem

Sometimes body can be not only string, but some complex objects-generators like request_toolbelt's MultipartEncoder.

In that case following error will be thrown:

TypeError: object supporting the buffer API required

when trying to calculate body hash here:

aws-requests-auth/aws_requests_auth/aws_auth.py

Line 146 in b86907f

payload_hash = hashlib.sha256(body).hexdigest()

where body is <MultipartEncoder: <generator object MultiValueOrderedDict.items at **0x7f7e9c69c970>>

How to test yourself:

pip install httpie httpie-aws-authv4
http --auth-type aws4 --auth REDACTED.execute-api.us-west-2.amazonaws.com \
     --multipart POST http://example.com/test field=value

Implementation notes

In case of MultipartEncoder, I had to materialize request body to string and replace request body with it, otherwise HTTPie would hang up undefinitely. I'm not good at Python, but suppose it is because we had emptied generator object for hash calculation and HTTPie waits for data from it to send in body but data would never come again.

[njedema]

Nice fix @Envek!
@DavidMuller can you please merge this? Had to write my own impl because 0.4.3 doesn't work for any non-string payload (i.e. JSON)