Fix bug https://github.com/ORIGYN-SA/CanDB/issues/28

[vporton]

This is an alleged fix of the bug #28. But it seems that the fix does not work :-(

Please, help with testing.

[vporton]

Now it is tested by https://github.com/vporton/bug-candb-delete well. But we need to add automatic tests (with all variants: adding an attribute to an empty value, adding an attribute to a value with another attribute, adding an attribute to a value having the same-named attribute. We also should test when all attributes are removed.

A comment in the submission shows a discrepancy between the documented behavior and real behavior of RT.update. This needs to be cleansed (and possibly can speedup code). I don't quite understand how your tests work.

My code is sub-optimal in cycles used. Please, speedup it. But most importantly, test all variants.

[vporton]

I've removed WIP from the title of the PR, because it is a critical issue and tests seems to have it worked. Right now, we can ignore that it has suboptimal speed, more important that it fixes a critical bug.