fix(tonic-web): fix dropped trailers in combined streaming call.

[zakhenry]

Motivation

In testing grpc-web streaming from AWS lambda I discovered a bug in the trailer frame parsing where if the upstream service combined data frames together (message + trailers in the same frame), the trailers would be dropped.

Solution

In tonic web frame decoding logic, ensure trailers are emitted on poll even when the data frame has been emitted.

[zakhenry]

note the CI failure is

  error: unused import: `pin`
  Error:  --> tonic/src/transport/server/io_stream.rs:6:11
    |
  6 |     pin::{pin, Pin},
    |           ^^^

which is not related to my diff.