context: fix original context not being restored

[EricFromCanada]

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Fixes #17641: if Thread.current[:context] returns nil, the original context won't be saved and restored, so use self.current which accounts for that possibility.

[MikeMcQuaid]

Good catch, thanks @EricFromCanada!

[Bo98]

Makes sense. Though for the Thread.current[:context] = old_context in the ensure block we do want the original thread-context rather than the global one so that Context.current = whatever works after a with_context.