imp(flows) don't require present_user_url to return an additional char

For custom AuthenticatorDelegate implementations it may be very weird to
be required to return an additional character after the code from
present_user_url.

One may find himself chasing the bug which is bad UX for the user.

Improve this behavior and therefore introduce a breaking change that
will hopefully not break existing implementations nevertheless.

src/authenticator_delegate.rs

@@ -137,7 +137,11 @@ pub trait AuthenticatorDelegate {
                      url);
 
             let mut code = String::new();
-            io::stdin().read_line(&mut code).ok().map(|_| code)
+            io::stdin().read_line(&mut code).ok().map(|_| {
+                // Remove newline
+                code.pop();
+                code
+            })
         } else {
             println!("Please direct your browser to {} and follow the instructions displayed \
                       there.",