fix EditController._ordered_formparams method in case some mandatory relation isn't specified
If you have an HTML form which doesn't carry information about some mandatory
relation (maybe because this relation is set by a hook), you ends up with a
KeyError when the form is posted.
Using a get instead of subscription protocol fixes the problem.
Closes #5481617.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html