HTTPClient should fail early if the internet connection is offline or disconnects #826
Description
Another big bump in a transition from URLSession to HTTPClient:
HTTPClient has no way of knowing the internet is offline or is disconnected. An executed request that hasn't connected will just result in a connection timeout, and a connected request that becomes disconnected during read/write will result in a read/write timeout, which with the shared singleton configuration is 90 seconds (!!!).
As a comparison, URLSession's default configuration causes a request to fail immediately with NSURLErrorNotConnectedToInternet unless waitsForConnectivity is set to true, in which case it then waits the full timeoutIntervalForRequest before failing, equivalent to AHC's Timeout.connect. No matter what waitsForConnectivity is set to, a disconnection from the internet during a connected request's read/write results in an immediate failure with NSURLErrorNetworkConnectionLost
I think URLSession's default behavior is quite sensible, and it would be great to offer an equivalent in AHC.