Added an optional 'headers' arg in HttpRpcTransport.
Made changes based on the review.
Changed finish_request() to accept a list of header values for a particular header field.
Fixed spelling error in HttpRcpTransport test.
Merge branch 'develop' into feature/issue-455-optional-response-headers-in-httprpctransport
Merge branch 'feature/issue-455-optional-response-headers-in-httprpctransport' into develop
Merge branch 'develop' into feature/issue-446-iterable-model-instances