Fix bug with unbinding when component is unmounted

changelog.txt

@@ -1 +1,2 @@
 feature - Upgraded Firebase dependency to 2.x.x.
+fixed - Fixed bug which caused `unbind()` to be called on a previously unbound reference when a component was unmounted.

src/reactfire.js

@@ -351,8 +351,8 @@
           this.firebaseRefs[bindVar].off(event, offListener);
         }
       }
-      this.firebaseRefs[bindVar] = undefined;
-      this.firebaseListeners[bindVar] = undefined;
+      delete this.firebaseRefs[bindVar];
+      delete this.firebaseListeners[bindVar];
 
       // Update state
       var newState = {};

tests/reactfire.spec.js

@@ -1104,5 +1104,40 @@ describe('ReactFire', function() {
 
       shallowRenderer.render(React.createElement(TestComponent));
     });
+
+    it('handles already unbound state when the component unmounts', function(done) {
+      var TestComponent = React.createClass({
+        mixins: [ReactFireMixin],
+
+        componentWillMount: function() {
+          sinon.spy(this, 'unbind');
+
+          this.bindAsArray(firebaseRef, 'items0');
+          this.bindAsObject(firebaseRef, 'items1');
+
+          firebaseRef.set({
+            first: { index: 0 },
+            second: { index: 1 },
+            third: { index: 2 }
+          }, function() {
+            this.unbind('items0');
+
+            shallowRenderer.unmount();
+
+            expect(this.unbind).to.have.been.calledTwice;
+            expect(this.unbind.args[0][0]).to.equal('items0');
+            expect(this.unbind.args[1][0]).to.equal('items1');
+
+            done();
+          }.bind(this));
+        },
+
+        render: function() {
+          return React.DOM.div(null);
+        }
+      });
+
+      shallowRenderer.render(React.createElement(TestComponent));
+    });
   });
 });