title: PersistedModel.create(Array) returns an object with numbered properties
body: <p>Below is a snippet of code that calls a Loopback API method with an array of values. The input values are correct, no errors are thrown on the API and the subscribe block runs as expected. </p>
  
  <pre><code>    const newStudentGroups =
              selectedStudentIds.map(sId =&gt; ({
                  studentId: sId,
                  groupId: this.group.id,
                  schoolId: this.curSchool.id,
                  programId: this.curProg.id,
              }));
     this.stuGroupApi.create(newStudentGroups)
         .subscribe((newStu) =&gt; {
            console.log(newStu.map(s=&gt;s[0]));
         });
  </code></pre>
  
  <p>However the values returned to the subscribe block as newStu appears as an object of the form:</p>
  
  <pre><code>{ 
    0:{
       studentId: 123,
       groupId: 321,  
       schoolId: 1,
       programId: 5
      },
    1:{
       studentId: 132,
       groupId: 322,  
       schoolId: 1,
       programId: 5
      },
    2:{studentId: 143,
       groupId: 331,  
       schoolId: 1,
       programId: 5
      }
  }
  </code></pre>
  
  <p>I need an array of StudentGroup objects the same as I put in. I recognize that I can forkJoin individual calls to the API but that seems like a lot of network traffic vs a single call that could/should run as a batch on the DB.</p>
  
  <p>I can't find much to suggest if I've done this wrong or if there's a canonical way to get this back into the form in which I sent it. Is this a bug? Am I running the call incorrectly? </p>
  
  <p>EDIT: I've been inspecting the actual network requests and it looks like Loopback is actually returning the array as required. So this has to be the SDK on my end somewhere. Does @mean-expert/loopback-sdk-builder convert arrays to objects intentionally or did I misconfigure something? I'm on version"@mean-expert/loopback-sdk-builder": "^2.1.0-rc.10.5".</p>