Add another case to cleaning up runs of letters

[NSoiffer]

In #333, document has bad MathML for $log_2 3$. Each letter is in its own mi. Normally MathCAT would correct that, but the last letter carries the subscript ($g_2$) so that MathCAT is currently not seeing the run.

MathCAT's cleanup should allow for a scripted final letter. If MathCAT determines that the run of mis should be merged, the merged result should be the new base of the script.