Revert "add in a http header and ensure we proxy it (bug 1003248)"

This reverts commit 62fe6b372b3dbaff887fa0091978162ac6ecf97a.
Merge pull request #257 from andymckay/1016112

this http header was in the wrong place (bug 1016112)
Allow multiple apps to share seller (bug 1011022)

- Remove unique constraint
- Test that multiple products can share a seller
Merge pull request #258 from JaredKerim-Mozilla/1011617

Allow multiple apps to share seller (bug 1011617)