BUGFIX N-N: modifying an N-N collection (Add/Remove) modified the other side of the relation for wrong objects.

Reproduction:

Prerequisities:
1. a, b1, b2 are SoodaObjects.
2. As and Bs are two sides of the same relation.
3. a is in relation with b1 and b2.

Reproduction steps:
1. Initialize b2.As, e.g. call b2.As.Contains(a);
2. a.Bs.Remove(b1);

Expected:
2a. a and b1 are no longer in relation.
2b. Relations of other objects are unaffected.

Actual:
2a. OK.
2b. NOK. a and b2 are no longer in relation as if b2.As.Remove(a) was called.
This only affects the current Sooda transaction, not the database.