fixed filterText to work with fields that are methods

Fixed the searchProvider so it is able to search a field that is a method of an object instead of just properties of an object.

In the following example columnDefs, getAge() would display fine in the grid, but the filter would not work on it. With these changes the filter will work.

var gridOptions =  {
    data: myData,
    columnDefs: [{ field: 'name' },{ field: 'getAge()' }]
}