Fix a bug in Query.cloneQuery() that can occur when using extended syntaxes
cloneQuery() relies of serializing the query and then parsing it back in.  For serialization it called serialize() which will result in an empty string
if the syntax is not one of the standard syntaxes that ARQ supports which then causes a QueryParseException to be thrown when trying to parse the
serialized form back in.  The commit changes the code to call toString() so extended query syntaxes can override toString() on their Query objects to
provide a valid serialization which can then be parsed back in

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1508136 13f79535-47bb-0310-9956-ffa450edef68