title: rmagick does not overwrite EXIF property
body: <p>I'm trying to reset the Orientation EXIF field in my image using rmagick 2.13.1 and ruby 1.9.3p194, but it doesn't seem to save.</p>
  
  <pre><code>img = ::Magick::Image::read(local_source_path).first
  img.get_exif_by_entry()
  =&gt; [["ColorSpace", "1"],
   ["ExifImageLength", "2448"],
   ["ExifImageWidth", "3264"],
   ["ExifOffset", "38"],
   ["Orientation", "6"]]
  </code></pre>
  
  <p>Orientation can also be acquired thru the properties</p>
  
  <pre><code>img.properties
  </code></pre>
  
  <p>outputs</p>
  
  <pre><code>{"date:create"=&gt;"2014-01-08T17:26:10-08:00",
   "date:modify"=&gt;"2014-01-08T17:26:05-08:00",
   "exif:ColorSpace"=&gt;"1",
   "exif:ExifImageLength"=&gt;"2448",
   "exif:ExifImageWidth"=&gt;"3264",
   "exif:ExifOffset"=&gt;"38",
   "exif:Orientation"=&gt;"6",
   "jpeg:colorspace"=&gt;"2",
   "jpeg:sampling-factor"=&gt;"2x2,1x1,1x1"}
  </code></pre>
  
  <p>I try to set Orientation property to <code>nil</code> &amp; save:</p>
  
  <pre><code>img['exif:Orientation'] = nil
  </code></pre>
  
  <p>Now,</p>
  
  <pre><code>img.properties
  =&gt; {"date:create"=&gt;"2014-01-08T17:26:10-08:00",
   "date:modify"=&gt;"2014-01-08T17:26:05-08:00",
   "exif:ColorSpace"=&gt;"1",
   "exif:ExifImageLength"=&gt;"2448",
   "exif:ExifImageWidth"=&gt;"3264",
   "exif:ExifOffset"=&gt;"38",
   "jpeg:colorspace"=&gt;"2",
   "jpeg:sampling-factor"=&gt;"2x2,1x1,1x1"}
  </code></pre>
  
  <p>which looks correct.</p>
  
  <pre><code>img.write(local_dest_path)
  </code></pre>
  
  <p>but when I use <code>get_exif_by_entry</code></p>
  
  <pre><code>img.get_exif_by_entry('Orientation')[0][1]
  </code></pre>
  
  <p>I still get 6 &amp; not <code>nil</code>.</p>
  
  <p>When I read the written file that was modified <code>local_dest_path</code>, I still get "6".</p>
  
  <p>Is there a <code>set_exif_by_entry</code> function that I'm missing? Any ideas?  Thanks</p>