XSRF token mismatch error when using the "create" form to update a link

[offbyone]

If I use the create form on /go/ to update a link that already exists:

Rather than an updated link (my preferred outcome), an intermediate form indicating a collision with an offer to overwrite (probably the right outcome) or an error indicating that the link is duplicated (least ideal), what I instead get is a plain text HTTP response "Invalid XSRF token"

This is sort of related to #130 -- I think it's the same issue, and the users haven't recognized that they are creating duplicates -- and #156, which is about "renaming" a link.

[zofrex]

Easily reproduced locally in dev mode. Looking into it!