Remove downcase! method from sanitize function in ContentTypeField

[RemcodM]

Fixes #1398

This change removes the sequence of code that downcases the Content-Type header value. If I should believe the specs, it should serve no purpose. Only 2 specs break because of the charset being returned in capital letters instead of downcased as before.

This fixes the problem is which encoding is broken by downcasing, such as the example with base64 provided in #1398

[ahorek]

nice find! I think you're right, the code should be removed. Could you add a test case according to your example to avoid further regressions?

[mikel]

Hi there @RemcodM could you please rebase this onto latest master? This will then trigger all the current specs we have.