Fix handling of request/response body for Python 3

In Python 3 request/response body is represented as `bytes`. This commit
adds implicit decoding from/to bytes before parsing json from it. Also
the same problem was fixed for the tests: for example, parsing response
from simulate_request requires decoding it to text string.

Change-Id: I0d210a9bc4e3f24b4440588c96e7c06ca2411b64
Partially-implements: blueprint py3k-support
Merge "Fix handling of request/response body for Python 3"