DjangoTestAdapter cannot deal with certain start_response invocations

The start_response definition of rest-framework's test utilities does not conform to the signature presented in the WSGI PEP:

start_response(status, headers)   # wrong
start_response(status, headers, exc_info=None)  # right

This makes a difference for the following invocation, which is valid according to the WSGI PEP but will break the TestAdapter:

start_response(status, headers, sys.exc_info())
start_response(status, headers, None)

It is unclear to me whether the second invocation is actually valid, but a lot of implementations accept it regardless.

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