browser: don't default save() to empty string
node: don't default save() to an empty string
debug: invoke save() at the beginning of enable()

We modify the `namespaces` argument directly for some reason,
which ends up saving input that ends up being invalid RegExp code
upon the next `load()`+`enable()` call combo.

Avoid that completely by just saving the input directly first thing.
Fixes browsers since they actually use the "saved" value.