Ensure output HTML is UTF-8

Carolyn:

> One issue: when I download a PDF of the HTML page, an ‘A’ appears if I don’t include a caption.

It turns out that `Â` is what [NON-BREAKING SPACE](http://www.fileformat.info/info/unicode/char/a0/index.htm) maps to when taken from UTF-8 format and interpreted as ISO-8859-1.

This patch fixes that problem by wrapping the input HTML in headers which indicate the file is UTF-8.