title: CakePHP3.4: Why Request::getHeader() returns an array?
body: <p>I have the following request:</p>
  
  <pre><code>2017-04-05 06:53:31 Error: Cake\Http\ServerRequest Object
  (
      ...
      [_environment:protected] =&gt; Array
          (
              [HTTP_REGISTRATION] =&gt; eur73_9lhfQ:...SJMryPxCNzKcqSufdpFMOaux
              ...
          )
  
      [_detectorCache:protected] =&gt; Array
          (
              [ajax] =&gt; 
              [get] =&gt; 1
              [head] =&gt; 
              [options] =&gt; 
              [post] =&gt; 
          )
  
      [uri:protected] =&gt; Zend\Diactoros\Uri Object
          (
              [allowedSchemes:protected] =&gt; Array
                  (
                      [http] =&gt; 80
                      [https] =&gt; 443
                  )
              ...
  
          )
      ...
  )
  </code></pre>
  
  <p><code>$this-&gt;request-&gt;header['registration']</code> returns:</p>
  
  <pre><code>eur73_9lhfQ:...SJMryPxCNzKcqSufdpFMOaux
  </code></pre>
  
  <p>as string</p>
  
  <p>$this->request->getHeader('registration') returns:</p>
  
  <pre><code>[Registration] =&gt; Array
      (
          [0] =&gt; eur73_9lhfQ:...SJMryPxCNzKcqSufdpFMOaux
      )
  </code></pre>
  
  <p>Why does it return an array?</p>