Run iri_to_uri on header values

[ericholscher]

This fixes #11403

[ericholscher]

@stsewd hrm, this is now breaking another test because it's adding X-RTD-Resolver-Filename in a place where it wasn't before, because presumably now it isn't invalid. That seems fine?

         self.assertNotIn("X-RTD-Resolver-Filename", r.headers)
         AssertionError: 'X-RTD-Resolver-Filename' unexpectedly found in {'Content-Type': 'text/html; charset=utf-8', 'Location': 'http://project.dev.readthedocs.io//example.com/path.html', 'X-RTD-Redirect': 'user', 'X-RTD-Domain': 'project.dev.readthedocs.io', 'X-RTD-Project': 'project', 'X-RTD-Version': 'latest', 'Cache-Tag': 'project,project:latest', 'X-RTD-Project-Method': 'public_domain', 'X-RTD-Version-Method': 'path', 'CDN-Cache-Control': 'public', 'X-RTD-Resolver-Filename': '/en/latest/%0D/example.com/path.html', 'access-control-allow-origin': '*', 'access-control-allow-methods': 'HEAD, OPTIONS, GET', 'Referrer-Policy': 'no-referrer-when-downgrade', 'X-Content-Type-Options': 'nosniff', 'Cross-Origin-Opener-Policy': 'same-origin', 'Content-Length': '0', 'Vary': 'Accept-Language, Cookie', 'Content-Language': 'en'}

[stsewd]

@ericholscher yeah, should be fine. We can probably also remove the call to ResponseHeaders({})._convert_to_charset now.