Headers added by requests aren't valued

[Dunedan]

In an OpenAPI specification I have some mandatory headers defined for certain operations. These headers are added by a custom requests authentication handler and have to be added there, because they are based on the content of the rest of the request (essential it's Signing HTTP Messages).

As bravado currently does the validation of headers without accounting for the headers requests adds, it claims that these headers are missing, while they are indeed there.

I consider that as a bug as the headers bravado is checking are different from the headers it's actually sending to the API.