fix: Search fails on casting ValueExpression

* Search fails on some occasions when casting to StringValue or
  UntypedValue.
  * The Search engine probably is incosistent when it uses UntypedValue
    vs StringValue

src/Spark.Mongo/Search/Searcher/MongoSearcher.cs

@@ -216,7 +216,7 @@ private Criterium CloseCriterium(Criterium crit, string resourceType, int level)
                 throw new ArgumentException(String.Format("None of the possible target resources support querying for parameter {0}", crit.ParamName));
             }
             crit.Operator = Operator.IN;
-            crit.Operand = new ChoiceValue(allKeys.Select(k => new StringValue(k)));
+            crit.Operand = new ChoiceValue(allKeys.Select(k => new UntypedValue(k)));
             return crit;
         }