[Feature Request] Throw a warning if kwarg "_class=..." is given

[Thelma Runyon]

When you only use bs4 occassionally, it's easy to forgot if the correct filter for functions like `soup.filter(...)` is `class_=...` or `_class=...`. If you provide `_class=...` instead of `class_=...`, then no results are returned and it's a serious hassle to debug.

Can BS4 throw a warning if someone tries to provide a `soup.filter(..., _class='some-class')` arg?

[Leonard Richardson (leonardr)]

I am OK with issuing a SyntaxWarning in this situation and otherwise maintaining the current behavior.

[Chris Papademetrious (chrispitude)]

This one bit me too, including the part about being difficult to debug because there's no obvious failure.

[Leonard Richardson (leonardr)]

Fixed in revision 1b679b7 on the 4.13 branch.