overwrite save() method on twinnable many many to use common ids instead...

... of ids

I assumed that key_to and key_from can not be arrays when using a twinnable many_many.

I'm not sure if coding style is okay as I pasted fuel code for many_many relation.

The explanation of this PR is that no relation were deleted when it was supposed to be done. Original ids were ids instead of common ids, which are used in the "in between" table.