Update form's model if the input is pristine and its value changes; test

This fixes a case where a form initializes with default values loaded
from the server.  If an update comes in from the server and the user has
not touched the field (its pristine) then update the field's value.

This feature will help track if the user has actually made any changes
from the default value that the form was/would have been initialized
with.

specs/Formsy-spec.js

@@ -120,6 +120,58 @@ describe('Formsy', function () {
 
     });
 
+    it('should allow a dynamically updated input to update the form-model', function (done) {
+
+      var forceUpdate = null;
+      var model = null;
+      var TestInput = React.createClass({
+        mixins: [Formsy.Mixin],
+        changeValue: function (event) {
+          this.setValue(event.target.value);
+        },
+        render: function () {
+          return <input value={this.getValue()} onChange={this.changeValue}/>
+        }
+      });
+
+      var input;
+      var TestForm = React.createClass({
+        componentWillMount: function () {
+          forceUpdate = this.forceUpdate.bind(this);
+        },
+        onSubmit: function (formModel) {
+          model = formModel;
+        },
+        render: function () {
+          input = <TestInput name='test' value={ this.props.value } />;
+
+          return (
+            <Formsy.Form onSubmit={this.onSubmit}>
+              {input}
+            </Formsy.Form>);
+        }
+      });
+      var form = TestUtils.renderIntoDocument(<TestForm value='foo'/>);
+
+      // Wait before changing the input
+      setTimeout(function () {
+        form.setProps({value: 'bar'});
+
+        forceUpdate(function () {
+          // Wait for next event loop, as that does the form
+          setTimeout(function () {
+            TestUtils.Simulate.submit(form.getDOMNode());
+            expect(model.test).toBe('bar');
+            done();
+          }, 0);
+
+        });
+
+      }, 10);
+
+    });
+
+
     it('should invalidate a valid form if dynamically inserted input is invalid', function (done) {
 
       var forceUpdate = null;

src/main.js

@@ -160,6 +160,15 @@ Formsy.Mixin = {
     nextProps._validate = this.props._validate;
   },
 
+  componentDidUpdate: function(prevProps, prevState) {
+    if (this.state._isPristine) {
+      if (this.props.value !== prevProps.value && this.state._value === prevProps.value) {
+        this.state._value = this.props.value || '';
+        this.props._attachToForm(this);
+      }
+    }
+  },
+
   // Detach it when component unmounts
   componentWillUnmount: function () {
     this.props._detachFromForm(this);