Fix Python3 bugs

This patch fixes three issues that were found running
functional tests with Python 3:

1. python-requests sets 'application/x-www-form-urlencoded' as
   content-type if the input is not a string object and no
   content-type is given.

2. Encoding of the headers is now only done if required. This
   prevents comparisons between unencoded headers and encoded
   prefixes and avoids unnecessary forloop-iterations.
   One unittest was extended to ensure it works for unencoded
   and encoded headers with or without the prefix.

3. Functional tests recently switched to using byte data for
   testing, thus the comparison needs to be a byte object as well.

Change-Id: I035f8b4b9c9ccdc79820b907770a48f86d0343b4