Implement SendFormUrlEncoded method for application/x-www-form-urlencoded form data

[dionrhys]

In #9, the SendForm extension method was changed from sending a application/x-www-form-urlencoded request to sending one as multipart/form-data instead. This is a breaking change in some cases if sending to an endpoint that doesn't expect form data to come in with multipart/form-data encoding (OIDC access token requests for example, see opserver/Opserver#426).

This PR re-adds the old behaviour as a new extension method so consuming libraries don't need to add it back themselves.

I expect application/x-www-form-urlencoded is the more common media type that consumers will want (when size limitations or file uploads aren't a concern), but I didn't want to revert SendForm back to its original behaviour to avoid breaking dependent projects again.

I could also obsolete SendForm and provide SendMultipartForm / SendFormUrlEncoded but I don't think it's important enough to inconvenience consumers with changing their code (and getting build errors using TreatWarningsAsErrors).

[NickCraver]

Looking good - we should add a test to ensure the form type on both (testing is in a pretty good state for adding here), happy to help tonight if not obvious!

[dionrhys]

Added tests!