Support binary payloads and responses, and non-UTF8 string payloads for Python functions

[jpauwels]

The Python templates currently don't support binary payloads and responses, whereas other templates do (e.g. image-handling functions in store). Much like #203 and #159 for Java, it would be nice to add this capability to the Python templates. Additionally, string payloads are assumed to be UTF-8 encoded, even if content-type signals differently.

Expected Behaviour

When passing binary payload, don't decode to string. When passing string payload, respect content-type encoding.
When returning binary file, don't encode as string.

Current Behaviour

UnicodeDecodeError when passing binary payload, incorrect decoding when passing non-UTF-8 string payload. Binary responses get stringified.

Possible Solution

I've got this fixed and working for more than a year in my custom template, happy to send a PR

Steps to Reproduce (for bugs)

Context

Using audio files as payload

Your Environment

• Docker version docker version (e.g. Docker 17.0.05 ):
  All
• Are you using Docker Swarm or Kubernetes (FaaS-netes)?
  Swarm
• Operating System and version (e.g. Linux, Windows, MacOS):
  Linux, MacOS
• Link to your project or a code example to reproduce issue: