Catch error and warn if unable to remove a method

[brandur]

In some cases there can be a method that's detected with
method_defined?, but which cannot be removed with remove_method,
even though it's on the same class. The only case so far that we've
noticed this is when a class is reopened for monkey patching like in
issue #749.

We'll still try to discourage this sort of use, but here we swallow the
error and issue a warning so at least the program doesn't crash.

Fixes #749.

r? @ob-stripe

[brandur-stripe]

Unfortunately I didn't add tests here because I don't know of a way to reproduce the problem that doesn't have side effects. I tested that it works with a separate repro script though.

[ob-stripe]

LGTM.

OOC, do you know what causes a method to have method_defined? == true but not be removable via remove_method?

[brandur-stripe]

OOC, do you know what causes a method to have method_defined? == true but not be removable via remove_method?

Ugh, I wish I understood this better, but unfortunately — no, not completely.

The normal case where this is possible is if a method is defined in an ancestor class. In this case it'll be reported as method_defined? == true, but you can't remove the method (although you can undef_method it to accomplish something similar).

That's not the case here though because it's the same class. There must be some subtlety that causes a similar effect when a class is reopened, but I couldn't find anything from reading the documentation.

Anyway, thanks for the review!