Tests to display the default em-dash when display_as returns nil

I tried and failed to reproduce the problem mentioned in: https://github.com/bernat/best_in_place/pull/272
However if this issue ever comes back due to a future commit, the specs I used to try and reproduce this problem can be useful. 

In this pull request:
* Removed duplicate trim method call
* Added a spec to test response being nil from display_as method
Yes, but shouldn't we show the placeholder when value is nil ?