Fix routing extensions leaking out of their scopes.

Each route builder should not leak extensions into other scopes. When
defining extensions in one route builder, they should not automatically
be available in other scopes. What we want to avoid is a plugin like
DebugKit adding `.json` extensions to all the application routes.

Refs #5089
Merge pull request #5114 from cakephp/issue-5089

Fix routing extensions leaking out of their scopes.