Updated the html2html filter. When we encounter a charset

declaration within the meta tags, the charset 'unicode' is manually aliased
to 'UTF-8'.  GNU iconv wants to treat it as an alias for 'UTF-16' but
messages generated by brain-damaged MS Exchange webmail are declaring
'unicode' when they really mean 'UTF-8'.  This change only applies to
meta tags; it is not a program-wide alias.