Review HTTP tests carefully

[jskeet]

Currently our ASP.NET Core integration test only works because when you create an HttpContent from an event without data, in binary mode, the formatter ends up creating content of "null" (as four bytes). If there's no data, we shouldn't be specifying a DataContentType or populating the request body - but that throws at the moment.

Likewise other HTTP tests specify a content type of XML despite using a JSON event formatter - it's not clear what the expected behaviour is there, in general. (That's related to #55.)

[jskeet]

The tests are a bit more sensible now, but they're not nearly comprehensive enough.