CORS implementation in servlet interferes with servlet container CORS implementation

I deployed the servlet in Tomcat 7.0.42. Similar problems were also observed in Tomcat 6.0.37.

Single origin requests to the servlet were successful. Cross origin requests failed silently - no response to client; no error on server.

The failure case was demonstrated by deploying the elasticsearch HEAD plugin as a webapp in a web container on a different port. The success case was demonstrated by merging the elasticsearch HEAD plugin into the webapp with the servlet. Both webapps were accessed with the Opera web browser and pointed to the same elasticsearch servlet.

Removing the CORS implementation from AbstractServletRestChannel allowed the Tomcat CORS filter to return the correct failure response to the client. Then configuring the Tomcat filter allowed CORS requests to succeed.

Note that the servlet CORS implementation always allows requests from any origin. By comparison, the Tomcat filter is configurable.

Closes #11
Closes #12