Fixing JIRA Issue #1611: https://issues.apache.org/jira/browse/XERCESJ-1611. Adding a null check to guard against an NPE that can occur when the content-type returned from the URLconnection is null.

git-svn-id: https://svn.apache.org/repos/asf/xerces/java/trunk@1478179 13f79535-47bb-0310-9956-ffa450edef68