Adding an option to avoid duplicate attribution.

Thanks Marc Jansen for this nice contribution.  Without wanting
to gush, I think this should be a model contribution because:

 1. it is a simple change
 2. with a nice example that clearly demonstrates the benefit
 3. it includes tests!
 4. and Marc wrote a note to the dev list with links to the ticket

r = me (closes #2266)



git-svn-id: http://svn.openlayers.org/trunk/openlayers@9720 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Control/Attribution.js

@@ -22,6 +22,12 @@ OpenLayers.Control.Attribution =
      * {String} String used to seperate layers.
      */
     separator: ", ",
+    
+    /**
+     * APIProperty: eliminateDuplicates
+     * {Boolean} shall already present attribution strings be eliminated when other layers have the same attribution string?
+     */
+    eliminateDuplicates: false,
        
     /**
      * Constructor: OpenLayers.Control.Attribution 
@@ -81,9 +87,13 @@ OpenLayers.Control.Attribution =
             for(var i=0, len=this.map.layers.length; i<len; i++) {
                 var layer = this.map.layers[i];
                 if (layer.attribution && layer.getVisibility()) {
-                    attributions.push( layer.attribution );
+                    // add attribution if duplicates shall be ignored generally
+                    // or if the current attribution string is unique
+                    if (!this.eliminateDuplicates || OpenLayers.Util.indexOf(attributions, layer.attribution) === -1) {
+                        attributions.push( layer.attribution );
+                    }
                 }
-            }  
+            } 
             this.div.innerHTML = attributions.join(this.separator);
         }
     },