fix filter aggregation for string type on the right side

[imansafari1991]

I try to use the filter on the aggregation as follows:

// Format Customer Full Names
            customerAggregations= customerAggregations.Apply(x => string.Format("{0} {1}", x.RecordShell.FirstName, x.RecordShell.LastName),
                "FullName");
            // Filter Customer whose full name is "Bruce Wayne"
            var filteredCustomers =
                customerAggregations.Filter(p => p.Aggregations["FullName"] == "Bruce Wayne").ToList();

But I've got an exception :

System.Exception: 'Syntax error at offset 19 near 'Wayne'
Failed on FT.AGGREGATE customer-idx * APPLY format("%s %s",@FirstName,@LastName) AS FullName FILTER @FullName == Bruce Wayne'

The translated query must add ' ' around the right-side expression. After more inspection, I found out p.Aggregations["FullName"] == "Bruce Wayne" is a unary expression and existing code, just add ' ' around the right-side when the left-side is a memberexpression.
Added a commit to solve.

Thanks.

[slorello89]

LGTM 👍