Correct cropping with ImageMagick

The crop was missing the apropriate resizing operation. The test didn't
catch it because it used the same height as the original.

src/ImageMagickAdapter.php

@@ -88,10 +88,14 @@ public function crop($width, $height)
         if ($width == 0) $width = $height;
         if ($height == 0) $height = $width;
 
+        $size = $width . 'x' . $height;
+
+        $this->args[] = '-resize';
+        $this->args[] = "$size^";
         $this->args[] = '-gravity';
         $this->args[] = 'center';
         $this->args[] = '-crop';
-        $this->args[] = $width . 'x' . $height . '+0+0';
+        $this->args[] = "$size+0+0";
         $this->args[] = '+repage';
         return $this;
     }

tests/BaseAdapterTest.php

@@ -63,10 +63,10 @@ public function testCrop()
         $this->assertSize($orig, 1000, 500);
 
         ($this->getAdapter($orig))
-            ->crop(500, 500)
+            ->crop(250, 250)
             ->save($dest);
 
-        $this->assertSize($dest, 500, 500);
+        $this->assertSize($dest, 250, 250);
         $this->assertColor($dest, 'top', 'blue');
         $this->assertColor($dest, 'bottom', 'green');
         $this->assertAlpha($dest, 'center', 100);