Minor fixes to LtiOutcomeService while debugging:

* Added XML escaping for lis_result_sourcedid, as this is often not an
identifier like the spec suggests. (Hello Moodle!)
* Fixed an incomplete log message added in previous commit.
Further improvements to LtiOutcomeService:

* LIS result sending method is now independent of the rest of the
QTIWorks Engine machinery, so can be invoked easily if debugging of
TCs is necessary.
* HTTP request now includes Content-Type and Content-Length headers