Merge HTML detection to text detection service

Setting the filter name explicitly is not really required,
because TypeDetection::impl_checkResultsAndAddBestFilter
is able to select the correct filter anyway. But it seems that
other detection services also do it, so I followed that way.

Change-Id: I6e73fa79c6867d82f98d63e8d2b7865446f088ef
Reviewed-on: https://gerrit.libreoffice.org/8213
Reviewed-by: Kohei Yoshida <libreoffice@kohei.us>
Tested-by: Kohei Yoshida <libreoffice@kohei.us>