title: Function imwrite on imageio (Python) seems to be rescaling image data
body: <p>The function imwrite() on imageio (Python) seems to be rescaling image data prior to saving. My image data has values in the range [30, 255] but when I save it, it stretches the data so the final image spreads from [0, 255], hence creating "holes" in the histogram so as increasing overall contrast.</p>
  
  <p>Is there any parameter to fix this and make imwrite() not to modify the data?</p>
  
  <p>Thanks</p>
  
  <p>So far I am setting a pixel to 0 to prevent this from happening:</p>
  
  <pre><code>prediction[0, 0, 0] = 0
  </code></pre>
  
  <p>(prediction is a [1024, 768, 3] array containing a colour photograph)</p>
  
  <pre><code>imageio.imwrite('prediction.png', prediction)
  </code></pre>