Change RestRequest.Method type from enum to HttpMethod

[alexeyzimarev]

Currently, the type of RestRequest.Method property is RestSharp.Method. Before making the call, RestSharp converts the enum value to a HttpMethod instance. The enum is not extendable, so it is impossible to use any other method that is not included to the enum. Which leads to issues like #1048 and #2124. Whilst the first issue was obvious to implement, adding more and more enum values for not commonly used method possesses a challenge for maintainability.

It could be a good idea to allow using HttpMethod directly, but there's another challenge of backwards compatibility. Also, RestSharp provides enum values for methods like PATCH (n/a in .NET Framework) and MERGE, COPY and SEARCH, which aren't available in HttpMethod as constants.

I'd be happy to hear any design suggestions that would allow using any custom method without changing RestSharp code.