[ZF-8825] Lower-case lookup for "authorization" header

- A number of libraries, including CordovaJS, send the Authorization
  header as "authorization" (all lowercase); added a conditional check
  for this.
- Cannot test this, as it depends on the output of
  apache_request_headers().

See http://framework.zend.com/issues/browse/ZF-8825 for a related ZF1 issue, and https://issues.apache.org/jira/browse/CB-1455 for a related CordovaJS issue.