Github Issue:1178 -- throw warning with trying to set readonly attribute

Inspired from this issue: https://github.com/jnicklas/capybara/issues/1178

This is my first PR to an open source project, so please correct me if I didn't follow some protocol, I'd like to contribute more in the future.

I decided to throw a warning instead of throwing an error (As the issue requested) for backwards compatibility

I did not create any new tests, but modified the 2 tests I could find that cared about read only fields.