MDL-20808 Fixes for amf web services and test client - a web service browser.

MDL-20808 Fixes for amf web services and test client - a web service browser.

Fixed the bugs that Jerome mentions in the comments for this issue :

* No exception thrown when there is no return value now.
* No exception thrown when there are extra keys in the return value that are not in the description. Now the extra keys are ignored and not returned which is what the other web services do.
* Improved information about exceptions in the test client.