Add special casing of Data return type

[joshuawright11]

This makes it so you can just return Data or Data? from your API function to easily access the raw response body. The first will throw an error if the response body is empty, the second will successfully return nil.

Issue #20.

@GET("/body")
func getBody() async throws -> Data

@GET("/optional_body")
func getOptionalBody() async throws -> Data?