[BUGFIX] Deliver WSDL with /ter?wsdl

The SOAP endpoint does not deliver the WSDL file when using the common ?wsdl GET parameter.

This is now added, allowing to use

http://extensions.typo3.org/ter?wsdl

in addition to:

https://extensions.typo3.org/typo3conf/ext/ter/tx_ter_wsdl.php

to conform standards (which are also necessary for the tests)