Fix type of aiohttp raw headers

[rsmeral]

Description

aiohttp's ClientResponse.raw_headers is supposed to be "unconverted bytes".

Pook was setting the keys and values of raw_headers as string instead of bytes.

This was causing issues in clients which expect the headers to be bytes, for example aiobotocore tries to decode() header names and values which then fails with AttributeError: 'str' object has no attribute 'decode'.
See https://github.com/aio-libs/aiobotocore/blob/master/aiobotocore/endpoint.py#L41-L56

PR Checklist

• I've added tests for any code changes
• I've documented any new features

[sarayourfriend]

Thanks again @rsmeral. I'll review this later today or sometime tomorrow 🙂