Fix for bug #1170: slice of undefined

This code fix the bug #1170 by allowing more uses of `$filters` when searching the grid using the property `cellFilter` in column definitions.
Originally, `cellFilter` only accepted filters described as: 
`cellFilter: "<angular-filter-name>:'<filter-param>'"`
It was impossible to use a filter without a parameter or with parameters that were not single quoted.
Now, it is posible to define `cellFilter` such as:
- `cellFilter: "date:'dd-MM-yyyy'"`
- `cellFilter: "date"`
- `cellFilter: "someOtherFilter:true"`