add content negotiation middleware and tests

server/middleware/content-negotiation.js

@@ -0,0 +1,12 @@
+module.exports = (req, res, next)=>{
+	const isImageRequest = req.get('Accept').split(',')
+        .filter((h)=>!h.includes('q='))
+        .every((h)=>/image\/.*/.test(h));
+	if(isImageRequest) {
+		return res.status(406).send({
+			message : 'Request for image at this URL is not supported'
+		});
+	}
+
+	next();
+};