soup-request-http: fix usage with non-default-context

soup-request-http: fix usage with non-default-context

SoupHTTPInputStream was doing I/O in a thread if the session didn't
use the global default context. But really, it should have been
checking the thread-default context instead. And anyway, the threaded
version doesn't actually work, it turns out. So, fix the check, make
it into a g_return_if_fail(), and remove the threaded codepath.

Also, fix a handful of places that were adding sources to the global
default context rather than the thread-default/SoupSession context.

Add tests/requester-test to do some basic
SoupRequester/SoupRequestHTTP/SoupHTTPInputStream testing.

https://bugzilla.gnome.org/show_bug.cgi?id=653707