ipn/localapi: fix inability to receive taildrop files w/ escaped names

The localapi was double-unescaping: once by net/http populating
the URL, and once by ourselves later. We need to start with the raw
escaped URL if we're doing it ourselves.

Started to write a test but it got invasive. Will have to add those
tests later in a commit that's not being cherry-picked to a release
branch.

Fixes #2288

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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