Update jquery.populate.js

In case of non-presence of a field with a name like this 'my_form[myfield][]' (
<input type="checkbox" name="my_form[myfield][]" />). The jquery returns an error. 
To fix the problem, we need to escape the selector.
Merge pull request #1 from franek/master

Fix a problem when a field is not present in the form