stub_const polishing

[fxn]

Just some polishing:

• Fixes the assert_equal line in the documentation, should not have an assignment.
• Constants belong to class and module objects. Since Class < Module, the conventional name for the first parameter is mod.
• The code works under the assumption that the constant belongs to the first argument. For example, remove_const would not work otherwise, and also you const_set on that object. To be consistent with this, we pass a false second argument to const_get.
• Adds a test that specifies what should happen if the constant does not exist in the receiver.