Allow `Content-Type` header value be of ContentType class

It looks like somewhere in the code that uses Flanker we set the Content-Type header to a value of ContentType class. I have not been able to find that place but an exception traceback clearly states that it is indeed what happens. This is not a good thing to fix errors of a code that uses a library by modifying the library itself, but nethertheless I propose this change to stop those errors occurring everyday from happening as soon as possible.