🎨 Style: fix missed boolean fields and accidental side-effects

- Rename abandoned -> isAbandoned (ctx.go)
- Rename userContextSet -> isUserContextSet (ctx.go, ctx_test.go)
- Revert error string to original (sse: stream closed)
- Revert isHostOnlyOrigin/isHostOnlyCookie to hostOnlyOrigin/hostOnlyCookie
  (non-boolean variables, cookiejar_test.go)