title: cv2.imread: how to keep color channel when reading in grayscale image
body: <p>i am trying to load an grayscale image with opencv2, but somehow the color channel gets completely removed. </p>
  
  <p>When i am doing: </p>
  
  <pre><code>img = cv2.imread(PATH_TO_IMAGE, 1) // load image as rgb 
  </code></pre>
  
  <p><code>img.shape</code> returns for example <code>(28,28,3)</code> </p>
  
  <p>But if i am loading the same image as grayscale: </p>
  
  <pre><code>img = cv2.imread(PATH_TO_IMAGE, 0) // load image as grayscale 
  </code></pre>
  
  <p><code>img.shape</code> returns <code>(28,28)</code>, but i would expect/need a shape of <code>(28,28,1)</code> </p>
  
  <p>Is this a bug in opencv or am i missing something? </p>
  
  <p>Thanks in advance! </p>