Create Rx extensions mapping to Alamofire response method

[JerryTheIntern]

Use Case:
Imagine an HTTP API which returned status code 200, with an empty body.
Using responseJSON or responseData produces:

could not be serialized. Input data was nil or zero length

Just using RxAlamofire's AF.rx.request(urlRequest: urlRequest).validate() works, but if the API returns a 500, the validation doesn't actually work because the response is not yet set on the response property of the DataRequest.

It seems that API's with empty bodies aren't considered bad practice, nor discouraged by the HTTP specification.
https://softwareengineering.stackexchange.com/questions/211275/should-an-http-api-always-return-a-body

The Change
Alamofire already has an API which returns just the raw HTTPURLResponse, this PR is just creating an Rx mapping for it.

[rxswiftcommunity]

	Warnings
⚠️	It looks like code was changed without adding anything to the Changelog. If this is a trivial PR that doesn't need a changelog, add #trivial to the PR title or body.

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