docs(API): fix the mess-up in the nested middleware
fix(Request): Improve HTTP_RANGE support

 - Add a test to check whether "bytes=" is supported
 - add a simple way to support the "bytes=" prefix by stripping it if present
This should not break backward compatibility.

Fix #380
Merge pull request #379 from jeffrey4l/wrong_doc

Fix the wrong doc in the falcon.API
Merge pull request #382 from Joacchim/master

Improve HTTP_RANGE support, Fix #380