Make cookies return null when the key doesn't exist.

src/Microsoft.AspNet.Http/RequestCookieCollection.cs

@@ -48,15 +48,15 @@ public string this[string key]
 
                 if (Store == null)
                 {
-                    return string.Empty;
+                    return null;
                 }
 
                 string value;
                 if (TryGetValue(key, out value))
                 {
                     return value;
                 }
-                return string.Empty;
+                return null;
             }
         }
 
@@ -232,4 +232,4 @@ public void Reset()
             }
         }
     }
-}
+}