AutoMarshallingHTTPClient __init__ bug

[Nathan Buckner]

When instansiating the AutoMarshalling client it uses the serialize_format to set the Content-Type but ignores the deserialize_format in terms of the Accept header requiring users to set it __init__ is called or serialization will fail.(unless the api defaults to the type the user sent in deserialize_format. Also it is possible to have a content_type that does not match the format used by the client.(application/{0}) Examples application/atom+xml.

Other known problems.
Since default_headers is not a CaseInsensitiveDict, setting content-type wont change the content type because of the capitalization.

class AutoMarshallingHTTPClient(HTTPClient):
    """@TODO: Turn serialization and deserialization into decorators so
    that we can support serialization and deserialization on a per-method
    basis"""
    def __init__(self, serialize_format=None, deserialize_format=None):
        super(AutoMarshallingHTTPClient, self).__init__()
        self.serialize_format = serialize_format
        self.deserialize_format = deserialize_format or self.serialize_format
        self.default_headers = {'Content-Type': 'application/{format}'.format(
            format=serialize_format)}

Proposed solutions:
1.
Remove editing default_headers from client
Making all users set the content-type and accept headers.
2.
Add Accept header to default headers
Expect users to make overrides for special cases ie. application/atom+xml

3. Add a headers override in the init