Headers validated case sensitive

[Dunedan]

When supplying headers using _request_options bravado's validation checks their names with case sensitivity, while it shouldn't, as HTTP header names are case insensitive.

That leads to SwaggerMappingError's when a header is required and the case sensitivity of its name differs between the OpenAPI specification and the provided header name in the code.

A workaround is to use a CaseInsensitiveDict instead of a dict for the headers in _request_config like:

from requests.structures import CaseInsensitiveDict

Pet = client.get_model('Pet')
Category = client.get_model('Category')
pet = Pet(id=42, name="tommy", category=Category(id=24))
swagger_client.pet.addPet(
    body=pet,
    _request_options={"headers": CaseInsensitiveDict(data={"foo": "bar"})},
).result()

While that workaround works fine I'd expect bravado to handle that internally in a transparent way for the user.