Wrong index signature on HTMLSelectElement

[inad9300]

The type definitions for HTMLSelectElement contain the following index signature: [name: string]: any.

The first thing to point out is that I do not find documentation about this index signature, neither on MDN nor in the spec. It does seem to work at least on Firefox, though. If it does what I think, the index signature should point to HTMLOptionElement | HTMLOptGroupElement (alternatively, HTMLElement, if you want to maximize convenience in some way), as these are the only two possible children of a <select> element.

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.