views: Make 'patch_to_mbox' return a string

views: Make 'patch_to_mbox' return a string

The 'patch_to_mbox' function returns an object which is coverted to a
string in all places where this call occurs. The string conversion
differs between Python 2 and 3 and while it has been updated in one
place, it was missed in two others. Resolve these issues and ensure they
don't happen again by returning strings from 'patch_to_mbox' instead.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Daniel Axtens <dja@axtens.net>