httputil: Fix support for non-latin1 filenames in multipart uploads

httputil: Fix support for non-latin1 filenames in multipart uploads

The change to be stricter about characters allowed in HTTP headers
inadvertently broke support for non-latin1 filenames in multipart
uploads (this was missed in testing because our i18n test case only
used characters in latin1). This commit adds a hacky workaround without
changing any APIs to make it safe for a 6.5.1 patch release; a more
robust solution will follow for future releases.

Fixes #3502