Fix ServerShell not being able to handle plugin assets.

Fixes #3575
Fix issue with inputs() and plugin models.

inputs() should not trigger errors when generating inputs for plugin
models. Previously the internal state of FormHelper was incorrect as
model() returned the plugin name instead of the modelname.

Fixes #3571
BC fixes for habtm. Closes #3545.

Signed-off-by: Rachman Chavik <rchavik@xintesa.com>
Merge branch '2.2'
BC fixes for disabled checkboxes

see: d504642
see: #3545

Signed-off-by: Rachman Chavik <rchavik@xintesa.com>
added test to disprove #3588