title: Multiple Objects into HttpParams
body: <p>I have some categories in a formcontrol, I send them in an array of string like this: </p>
  
  <blockquote>
    <p>[1,4,6]</p>
  </blockquote>
  
  <p>And that is my actual code:</p>
  
  <pre><code>let categoryIds = new Array&lt;String&gt;()
  this.selectedCategories.forEach((value: string, key: string) =&gt;
    categoryIds.push(key))
  
  let requestOptions = {
      params: new HttpParams()
          .set('title', this.createNewForm.controls['title'].value)
          .append('content', this.createNewForm.controls['content'].value)
          .append('categoryids', categoryIds.toString()),              
      withCredentials: true
  }
  </code></pre>
  
  <p>But I want to send them as an array of objects, with the old version of angular Http I was able to do a foreach of the object and append every category.
  But I don't know how to get every category and make each one an append to params.
  I need to get like this:</p>
  
  <blockquote>
    <p>...categoryId=1&amp;categoryId=4&amp;categoryId=6...</p>
  </blockquote>