title: Unable to set value dynamically for Lightning:InputField
body: <p>I have created a lightning component with lightning record edit form and a couple of fields. Wrote a Javascript method such that When I change the first field to a particular value then second field is updated. Below is my code.</p>
  
  <pre><code>&lt;aura:component implements="force:appHostable"&gt;
     &lt;aura:attribute name='test' type='String'/&gt;
     &lt;lightning:recordEditForm 
                            aura:id="editForm"
                            objectApiName="Case"
                            recordTypeId="{XXXXXXXXXXXXX}"&gt;
      &lt;div class="slds-col slds-size_1-of-2 slds-p-around_x-small"&gt;
          &lt;lightning:inputField aura:id="subject" fieldName="Subject" onchange="{!c.subjectChanged}"/&gt;
      &lt;/div&gt;
      &lt;div class="slds-col slds-size_1-of-2 slds-p-around_x-small"&gt;
          &lt;lightning:inputField aura:id="description" value="{!v.test}" fieldName="Description"/&gt;
      &lt;/div&gt;
  &lt;/lightning:recordEditForm&gt;
  &lt;/aura:component&gt;
  </code></pre>
  
  <p>Below is the Controller code</p>
  
  <pre><code>({
      subjectChanged : function(component, event, helper) {
          var subjectValue = component.find("subject").get("v.value");
          if(subjectValue === "U") {
              //var descriptionValue = component.find("description");
              //descriptionValue.set("v.value","User");
              component.set('v.test','User');
          }
  
      }
  })
  </code></pre>
  
  <ol>
  <li>Updated Subject field to U, then Description got updated to 'User'</li>
  <li>Update the Description field value to XYZ or clear the value.</li>
  <li>Change the subject field to X and then update back to U. </li>
  <li>Notice that JS method got executed, but description is not updated to 'User'.</li>
  </ol>
  
  <p>Whenever, user updates subject field to U, I want the description to be set to User. Any thoughts on this?</p>