StartTLSProtocol: Fix dual use of filter_preferred_suites() arguments. filter_preferred_suites() is used as a Variable.Variable change callback, in which case it will be called with a Variable.Variable as the first argument. This conflicted with the filter_preferred_suites() as it expected a SSL.SSLContext as the first argument. This caused errors like "Calling the NULL value.". Move the SSL.SSLContext to the second argument. Fixes [WS-618].