Description:
------------
When exportImagePixels is calling with TIFF 16bit grayscale picture, an array with all elements = 0 is returned for map = R,G,B,O,I,P and max value (eg 65535 for INTEGER, 255 for CHAR) for map = A. 





Test script:
---------------
$im = new Imagick();
$im->readImage('picture.tif');
$arr = $im->exportImagePixels(10,10,10,10,'I',imagick::PIXEL_QUANTUM);
var_dump( $arr );

Expected result:
----------------
array with INTEGER or FLOAT values depend on 6th parameter

Actual result:
--------------
array(16) {
  [0]=>
  int(0)
  [1]=>
  int(0)
  [2]=>
  int(0)
  [3]=>
  int(0)
  [4]=>
  int(0)
  [5]=>