9e89e71e broke uploadContent, making it set 'json=true' on the request, so that
we would try to turn raw content into JSON. It also misguidedly set a
client-side timeout of 30s.
Fix that, and add some tests to check uploadContent works.
In mock-request: distinguish between an expectation (ExpectedRequest)
and an actual request (Request). Add support for checking the headers, and the
request options in general, to Request.
Add a 3s delay between scrollback requests if the previous scrollback request
failed.
Return the same promise if scrollback() is called multiple times whilst a
scrollback request is ongoing.
