Fix return format of Response._response

[erakli]

This change needed for python3 code to work. Need help from more experienced folks to correctly test this.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[codecov-io]

Codecov Report

Merging #561 into master will increase coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
+ Coverage   65.53%   65.57%   +0.04%     
==========================================
  Files         103      103              
  Lines       25568    25568              
==========================================
+ Hits        16755    16767      +12     
+ Misses       8813     8801      -12

Impacted Files	Coverage Δ
src/saml2/httputil.py	0% <0%> (ø)	⬆️
src/saml2/__init__.py	86.52% <0%> (+0.18%)	⬆️
src/saml2/validate.py	79.21% <0%> (+4.31%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ef65dbf...aeff86b. Read the comment docs.