proxy: fix case order in a type switch

[quasilyte]

Type switch executes its clauses sequentially until the first match. The *tcpmux.Stream type implements net.Conn, so if net.Conn goes before *tcpmux.Stream case, it will catch *tcpmux.Stream values and will never let case *tcpmux.Stream` to execute. The solution is simple, move specific type case before the interface it implements.

Found using gocritic linter, caseOrder checker.