Escape double-quotes in filname, in Content-Disposition header

Bug 1578512: As specified in RFC 6266, the filename is a
"quoted-string", and as specified in RFC 2616 double quotes
within a quoted-string should be escaped with a backslash.

Change-Id: Id9d069a976406a82a6f0b6db92c696f700e00469
behatnotneeded: Can't test file uploads in behat yet
(cherry picked from commit aa8c6760)