InfravisionHelpsKeepThingsClean - Adding InternalsVisibleTo for Moq to allow keep internal methods internal.

One of the WebSocketHandler.SendAsync() overloads was made public only to enable testing. Granting InternalsVisibleTo to Moq makes it possible to revert this change. (The main motivation for hiding the method is that it takes a parameter that is not available on all platforms and therefore a lot of effort would be required to make the APIs consistent across platform without any apparent benefit since the method should not be public in the first place).

Also making WebSocketHandler class abstract since it is not really usable standalone so there is no need for it to be instantiatable.