Disposing of HttpClient and HttpMessageHandler

[alexeyzimarev]

This issue is covering the discussion of the commit that adds IDisposable interface to RestClient, and HttpClient disposal.

Here is the discussion: 95f1324#r64077599

[alexeyzimarev]

@jnyrup I think it should be fine in the latest preview.

[jnyrup]

Looks good.
I've got no further comments about the disposing of RestClient.

[alexeyzimarev]

Thank you for your review. I think it makes sense to make RestRequest sealed btw.