Potential fix for the content type header issue

[BarryCarlyon]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

This fixes #138

Description of Changes:

• parse the content type header using the mime library
• test the content type when present
• report the charset if a charset is returned in the content-type header

Example output:

✔ Valid response. Received challenge [omitted] in body
✔ Valid content-type header. Received type text/plain with charset utf-8
✔ Valid status code. Received status 200

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)