Allow user to customize literal encoding

As suggested by @mpriour, it would be nice if the user could determine how
date values are encoded in literal elements.  Extending this beyond date
values, we can provide an `encodeLiteral` method that can be overridden to
provide application specific behavior.

lib/OpenLayers/Format/Filter/v1.js

@@ -247,6 +247,25 @@ OpenLayers.Format.Filter.v1 = OpenLayers.Class(OpenLayers.Format.XML, {
         obj.filters.push(filter);
     },
 
+    /**
+     * APIMethod: encodeLiteral
+     * Generates the string representation of a value for use in <Literal> 
+     *     elements.  The default encoder writes Date values as ISO 8601 
+     *     strings.
+     *
+     * Parameters:
+     * value - {Object} Literal value to encode
+     *
+     * Returns:
+     * {String} String representation of the provided value.
+     */
+    encodeLiteral: function(value) {
+        if (value instanceof Date) {
+            value = OpenLayers.Date.toISOString(value);
+        }
+        return value;
+    },
+
     /**
      * Method: writeOgcExpression
      * Limited support for writing OGC expressions. Currently it supports
@@ -406,11 +425,8 @@ OpenLayers.Format.Filter.v1 = OpenLayers.Class(OpenLayers.Format.XML, {
                 });
             },
             "Literal": function(value) {
-                if (value instanceof Date) {
-                    value = OpenLayers.Date.toISOString(value);
-                }
                 return this.createElementNSPlus("ogc:Literal", {
-                    value: value
+                    value: this.encodeLiteral(value)
                 });
             },
             "LowerBoundary": function(filter) {