Don't append feature prefix twice in WFS requests

Some WFS servers like tinyOWS require the feature prefix to be in the
feature type for the feature to be found (eg `prefix:layer`).

The problem was, the prefix was always added to the feature type which
gave us a type name like `prefix:prefix:feature` or
`feature:prefix:feature`. The requests were then rejected by the WFS
server.

We now check if the feature type starts with the prefix. If it does, we
don't append it again. If it doesn't we do.

test/spec/ol/format/wfs.test.js

@@ -839,6 +839,47 @@ describe('ol.format.WFS', function() {
     });
   });
 
+  describe('when writing out a Transaction request', function() {
+    var text;
+    before(function(done) {
+      afterLoadText('spec/ol/format/wfs/TransactionMulti.xml', function(xml) {
+        text = xml;
+        done();
+      });
+    });
+
+    it('do not add feature prefx twice', function() {
+      var format = new ol.format.WFS();
+      var insertFeature = new ol.Feature({
+        the_geom: new ol.geom.MultiPoint([[1, 2]]),
+        foo: 'bar',
+        nul: null
+      });
+      insertFeature.setGeometryName('the_geom');
+      var inserts = [insertFeature];
+      var updateFeature = new ol.Feature({
+        the_geom: new ol.geom.MultiPoint([[1, 2]]),
+        foo: 'bar',
+        // null value gets Property element with no Value
+        nul: null,
+        // undefined value means don't create a Property element
+        unwritten: undefined
+      });
+      updateFeature.setId('fid.42');
+      var updates = [updateFeature];
+
+      var deleteFeature = new ol.Feature();
+      deleteFeature.setId('fid.37');
+      var deletes = [deleteFeature];
+      var serialized = format.writeTransaction(inserts, updates, deletes, {
+        featureNS: 'http://www.openplans.org/topp',
+        featureType: 'topp:states',
+        featurePrefix: 'topp'
+      });
+      expect(serialized).to.xmleql(ol.xml.parse(text));
+    });
+  });
+
 
   describe('when writing out a GetFeature request', function() {
     var text;