* .../subunit2html.py: Open the html file for writing in binary mode so
that we can specify utf8 as the encoding. Stop trying to decode strings
that have already been preprocessed by the subunit parser. Treat them as
strings and encode to bytestring when writing to the output file.
Change-Id: I81f13794ad7b40dfbc8ad44d2a9a5acd855c0029