RESTWS-392 - Workaround to disable addition of WWW-Authenticate header in response

For ticket - https://tickets.openmrs.org/browse/RESTWS-392

When a controller extending BaseRestController throws APIAuthenticationException and if the user is not logged in, WWW-Authenticate header is added to the response.

When browsers receives this header, a popup is displayed for entering BasicAuth credentials. This is not desirable when clients have their own login page. 

So as a workaround, i have added a check to look for "Disable-WWW-Authenticate" header in request and if present, not adding WWW-Authenticate header to the response. This change should not break any existing clients.