Connection instances have a readyState property, with corresponding ReadyState enum with values borrowed from XMLHttpRequest readyState spec. The readyState is useful for tests to easily determine when connections have been fulfilled or not. All use cases for watching readyState changes to provide a good user experience should be able to be satisfied with observers provided to Http, as well as the Observables/Streams returned from Http.

This property should be limited to MockConnection, renamed to state, and changed to reflect states that Http cares about, namely "Sent", "Downloading", and "Completed".