Use charset=utf-8 for text and JSON mimetypes

[valadaptive]

This matches what serve-static does: it uses the mime-types package, which always sets charset=utf-8 for mimetypes beginning with text/.

The other two mimetypes, application/json and application/manifest+json, are the only two standard non-text mimetypes that have charset: "UTF-8" explicitly set in mime-db.

Resolves #171.