fix underline in latex output

I don't know why underline gets a special treatment in the latex generator, but I found it to be buggy.  Here is an example input/output:

        <tei:p><tei:hi rend="underline">Den 5.<tei:hi rend="sup">ten</tei:hi> Januar. 1716.</tei:hi></tei:p>

        \uline{Den 5.\uline{\textsuperscript{ten}} Januar. 1716.}

As you can see, the underline occurs a second time in front of the sup, and this actually is rendered by a double underline, which is not intended.  The patch makes unederline like any other inline highlighter, which works for me.