Expose InnerResponse, Http properties

[piaste]

The current FSharp.Data.WebResponse wrapper does not easily allow the user to access the original response.

In particular, the WebResponse will normally (always?) be a HttpWebResponse, which exposes critical properties like StatusCode, which are currently buried pretty deep under type tests (and I might not have found them at all without debugging and looking at the source):

function 
| Error (e : Net.WebException) ->
    match e.InnerException with
    | :? Net.WebException as webExn ->
       match webExn.Response with
       | :? Net.HttpWebResponse r when r.StatusCode = Net.HttpStatusCode.Conflict -> 
         
            Log.Informationf("`Resource %s already exists, no operation needed", resourceName)
            Ok ()

This PR adds to FSharp.Data.WebResponse:

• a set of properties that wrap those in HttpWebResponse, returning None if the wrapped response isn't a HttpWebResponse

• an InnerResponse property that exposes the original WebResponse, for anything else a user might need

[dsyme]

Looks great. Request adding comments. Adding a test wouldn't go astray either....

[dsyme]

Could you put a /// comment on each of these? I know these are missing on the rest but we may as well not make the problem worse :)