Allow multiple addSourceType calls with same name

Since `addSourceType` adds the source type to a static map, it was
yielding an error if two map instances tried to add the same source
type.

This changes `addSourceType(name, SourceType)` to simply do nothing if
there is already a source type named `name`.

js/style/style.js

@@ -671,7 +671,7 @@ Style.prototype = util.inherit(Evented, {
 
     addSourceType: function (name, SourceType, callback) {
         if (Source.getType(name)) {
-            return callback(new Error('A source type called "' + name + '" already exists.'));
+            return callback(null, null);
         }
 
         Source.setType(name, SourceType);

test/js/style/style.test.js

@@ -1222,10 +1222,12 @@ test('Style#addSourceType', function (t) {
         style.addSourceType('bar', SourceType, function (err) { t.error(err); });
     });
 
-    t.test('refuses to add new type over existing name', function (t) {
+    t.test('does not overwrite existing source type, but returns gracefully', function (t) {
         var style = new Style(createStyleJSON());
+        var existing = _types['existing'];
         style.addSourceType('existing', function () {}, function (err) {
-            t.ok(err);
+            t.error(err);
+            t.equal(_types['existing'], existing);
             t.end();
         });
     });