title: graphicsmagick fails to get size when using gm(buffer,file)
body: <p>I'm using the node module gm, and I'm trying to identify the size of a PNG image. I've made the following small program to illustrate my issue. The size is retrieved if you use the gm constructor function that takes a file name. It fails if you pass a buffer + file name hint. </p>
  
  <p>The error returned is:</p>
  
  <pre><code>can't buffer image and identify size { [Error: Command failed: gm identify: No decode delegate for this image format ().
  gm identify: Request did not return an image.
  ] code: 1, signal: null }
  </code></pre>
  
  <p>Code sample:</p>
  
  <pre><code>var fs = require('fs');
  var gm = require('gm');
  
  var buffer = fs.readFileSync('./test/test_image.png',{encoding:'binary'});
  
  gm('./test/test_image.png').size(function(err,size) {
      if (err) console.log("can't identify from disk read");
      else {
          gm(buffer,'test_image.png').size(function(err,size) {
              if (err) console.log("can't buffer image and identify size");
              else console.log("all good");
              process.exit(0);
          }); 
      }   
  });
  </code></pre>
  
  <p>Any ideas on how to workaround this? My real use case is getting image data as base64 encoded from web POST endpoint, so reading from disk isn't really an option.</p>