Fix filter by date for the soap api

In the soap endpoint “mc_filter_search_issues”, it is possible to
filter the search result by date. At least it is defined in the wsdl.
But as described in the issue 0020101, this parameter has no
effect (yet).

Therefore handle the date parameters manually in the mc_filter_api,
so that it will be appended to the filter.

After this change it is possible to filter the search result by a
date range.

Fixes 0020101