nginx: fix type mismatch during compilation on macOS/OS X (#551)
nginx: fix type mismatch during compilation on macOS/OS X (#551)

* src/nginx/grpc_web_finish.c: Updated the `length` arguments for
  `EncodesGrpcCustomTrailers`, `EncodesGrpcMessage` and
  `EncodesGrpcStatusCode` to be `uint64_t` instead of `size_t` since
  `size_t` is not guaranteed to be the same size as `uint64_t`.
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