In response to the discussion kennethreitz/requests#1252, this PR would ensure that any calculated header key is returned as a native string.
Obviously, feel free to code review this. Let me know if you want me to go about this in a different way, or anything. =)
Tests for bad Content-Type type.
Content-Type should be returned as a native string, not a bytestring.
This commit ensures that all current tests that exercise the encoding
fail on Python 3, where native strings aren't bytestrings.
Return native strings for header-values.
Happy to help. =)