Fix create method response

Added the preserveWhiteSpace to the createMethodResponse method. Added a test for extracting a struct from response. Beware that this test will fail without the fix.

When a XmlRpc\Client->call() gets a XML-Response with whitespace,  createMethodResponse() extracts the values through ValueType\ObjectType. Without preserveWhiteSpace it will get 5 childNodes ['\n', 'name', '\n', 'value' '\n'] and will always fail to get the correct values from firstChild and lastChild.

Sorry, if I explaned it wrong in first pull request, making my first steps with github here.