Test trailers bug when there are data frames in queue

[tony612]

Test case for ninenines/cowlib#92

The failed test case can be seen:

Testing ninenines.cowboy-prs.req_SUITE: Starting test, 680 test cases
--
  |  
  | =ERROR REPORT==== 23-Dec-2019::13:13:14 ===
  | Ranch listener h2 had connection process started with cowboy_tls:start_link/4 at <0.8236.0> exit with reason: {{case_clause,{trailers,[{<<"grpc-status">>,<<"0">>}]}},[{cow_http2_machine,queue_data,4,[{file,"src/cow_http2_machine.erl"},{line,1339}]},{cow_http2_machine,send_or_queue_data,4,[{file,"src/cow_http2_machine.erl"},{line,1199}]},{cowboy_http2,maybe_send_data,4,[{file,"src/cowboy_http2.erl"},{line,770}]},{cowboy_http2,commands,3,[{file,"src/cowboy_http2.erl"},{line,634}]},{cowboy_http2,loop,2,[{file,"src/cowboy_http2.erl"},{line,247}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
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  | req_SUITE:do_trailers failed on line 1097
  | Reason: {badmatch,{error,{stream_error,closed}}}
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  |  
  | Testing ninenines.cowboy-prs.req_SUITE: *** FAILED test case 252 of 680 ***
  |  
  | =ERROR REPORT==== 23-Dec-2019::13:13:26 ===
  | Ranch listener h2c had connection process started with cowboy_clear:start_link/4 at <0.9485.0> exit with reason: {{case_clause,{trailers,[{<<"grpc-status">>,<<"0">>}]}},[{cow_http2_machine,queue_data,4,[{file,"src/cow_http2_machine.erl"},{line,1339}]},{cow_http2_machine,send_or_queue_data,4,[{file,"src/cow_http2_machine.erl"},{line,1199}]},{cowboy_http2,maybe_send_data,4,[{file,"src/cowboy_http2.erl"},{line,770}]},{cowboy_http2,commands,3,[{file,"src/cowboy_http2.erl"},{line,634}]},{cowboy_http2,loop,2,[{file,"src/cowboy_http2.erl"},{line,247}]},{cowboy_http,parse,2,[{file,"src/cowboy_http.erl"},{line,311}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
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  | req_SUITE:do_trailers failed on line 1097
  | Reason: {badmatch,{error,{stream_error,closed}}}
  | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |  
  | Testing ninenines.cowboy-prs.req_SUITE: *** FAILED test case 337 of 680 ***
  | Testing ninenines.cowboy-prs.req_SUITE: TEST COMPLETE, 678 ok, 2 failed of 680 test cases

[essen]

Merged via the other PR. Thanks!