Replace HTML entities in GridFieldAddExistingAutocompleter->doSearch() 

with actual charaters as the DropDown does not play nicely with HTML entities

this is what it looks like before this change:
![before](https://f.cloud.github.com/assets/186158/1499001/a2a81c34-484b-11e3-9ab7-63f70f0b3c01.png)

and after:
![after](https://f.cloud.github.com/assets/186158/1499002/a752c41e-484b-11e3-8c8b-0213d38a326d.png)