Http client expect

This Pull Request adds a feature to the HttpClient to turn off the Expect header that curl might sent if the payload appears to be too big. In a realtime context, this is a good idea to avoid an extra HTTP roundtrip even though the payload is big (> 1000 bytes) to keep a good latency.

Also, this Pull Request brings a new debug callback to the HttpClientCallbacks.
I would not move that.