add charset utf-8 header line to html redir
add charset utf-8 header line to html redir

This silences an error in Firefox:

> The character encoding of the HTML document was not declared. The document
> will render with garbled text in some browser configurations if the document
> contains characters from outside the US-ASCII range. The character encoding
> of the page must be declared in the document or in the transfer protocol.

wijnandhoitinga/nutils,nutils/nutils,CVerhoosel/nutils,joostvanzwieten/nutils