Log for which keys values were set to nil in deep_munge

I think we all want deep_munge to be gone soon, but I am afraid that we will have to wait little bit longer. There are many developers which are not aware of this behaviour so I thought that it would be nice of RoR if there was some kind of hint of what actually happened, so I added log message when value is set to `nil` in deep_munge.

I hope that with #13420 we will quickly make code from this PR go away, but in case it will take bit longer...

Any suggestions for better log message would be awesome.