Fix testsuite to work correctly with interoperability handshake

[rachmatowicz]

This PR does the following:

• fixes a typo in the system property used to enable EE interoperability mode in the testsuite
• defines a new method in the HttpTestServer to allow registering a singleton PathHandler to be wrapped (and so participate in the interoperability handshake) and registered as a service handler (c.f. /wildfly-services) in Undertow
• fixes an issue with handling UNAUTHORIZED responses to the first request across a connection which can cause the interoperability handshake to fail

For more details, see: https://issues.redhat.com/browse/WEJBHTTP-110

[fl4via]

@rachmatowicz I assume this is not ready for merge yet, given the comments you raised in the Jira, am I correct?
Still, I just want to let you know that, so far, it looks good to me. Thanks for the hardwork!