Add meta charset in head

The HTTP Content-Type header has precedence but adding a meta charset in the head helps when opening an offline (e.g. downloaded) file in the browser.

Also, note that the official character set name is in uppercase.
See: http://www.iana.org/assignments/character-sets/character-sets.xhtml
Merge pull request #435 from GeertDD/patch-1

Add meta charset in head