Dispose HTTP response in BbsClient and AdoClient

[ArinGhazarian]

I noticed that in BbsClient and AdoClient we are not disposing the HTTP response that is getting returned by the HttpClient. The response needs to be immediately disposed with a using statement and also all unit tests need to be fixed the way they were fixed in #925.

Todo

• [ ] Dispose the http response in BbsClient#SendAsync() and AdoClient#SendAsync() methods
• [ ] Fix unit tests by using a factory to return a new HttpResponseMessage when mocking HttpMessageHandler (see #925)
• [ ] Bonus: Simplify SendAsync method in both BbsClient and AdoClient by replacing the switch statement with HttpClient#SendAsync() the way it is in GithubClient#SendAsync().