reduce object creation during JSON serialization (#2576)

If a new {@code Double} instance is not required, this method
     * should generally be used in preference to the constructor
     * {@link #Double(double)}, as this method is likely to yield
     * significantly better space and time performance by caching
     * frequently requested values.

main/src/com/google/refine/util/SerializationFilters.java

@@ -109,7 +109,7 @@ public class SerializationFilters {
         @Override
         public void serialize(Double arg0, JsonGenerator gen, SerializerProvider s)
                 throws IOException {
-            if (new Double(arg0.longValue()).equals(arg0)) {
+            if (Double.valueOf(arg0.longValue()).equals(arg0)) {
                 gen.writeNumber(arg0.longValue());
             } else {
                 gen.writeNumber(arg0);