CMimeContentType::m_ParameterMap isn't modified after construction

Change-Id: Ib3dbfbd4212ed0d7e8a9f29c655f935d3fc4f1bf
Reviewed-on: https://gerrit.libreoffice.org/61698
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>