tapo: normalize fetch header case

[xdissent]

Fixes two-way audio on tapo cameras that require case-sensitive http headers.

[koush]

I think respecting the caller's initial casing is the better fix here:

https://github.com/koush/scrypted/commit/a807766699a5245ea7f550a1e6963b1163705634

[xdissent]

Ah, yeah that could work but needs to be done in authHttpFetch too. That creates a Headers immediately and then calls httpFetch() with just the Headers obj.