Issue #542

#542. request["SOME_HEADER"]
is a little confusing since the [] operator on request object is a 
shortcut to the params hash. And thus, request["key"] is same as 
params["key"]. Updated the same thing.

README.rdoc

@@ -1144,7 +1144,7 @@ error handlers) through the <tt>request</tt> method:
     request.host                # "example.com"
     request.get?                # true (similar methods for other verbs)
     request.form_data?          # false
-    request["SOME_HEADER"]      # value of SOME_HEADER header
+    request["some_param"]       # value of some_param parameter. [] is a shorcut to the params hash.
     request.referrer            # the referrer of the client or '/'
     request.user_agent          # user agent (used by :agent condition)
     request.cookies             # hash of browser cookies