Updated _content.py: encode_content to accept a bytearray non-streamed content

[jcekstrom]

Summary

Updated _content.py: encode_content to accept a bytearray as well as
bytes/str as a static content type. This allows sending a mutable bytearray
without having to cast to an immutable bytes object, trigging a memory
copy before sending.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.