fix: Parse accept header case-insensitively #162

Loadtest Results

param	v12.1	main	head
bytes_in.mean	1008	1007	1008
bytes_in.total	27707448	27890851	27954696
bytes_out.mean	3848	3849	3848
bytes_out.total	105795238	106557174	106768870
duration	59999730112	60000033173	60000809434
latencies.50th	968828	957906	952570
latencies.90th	2017032	1964689	2067934
latencies.95th	14099746	13989805	14028123
latencies.99th	14635204	14536045	14502285
latencies.max	28298835	24580030	25203618
latencies.mean	2181363	2166255	2161704
latencies.min	153708	154809	151855
latencies.total	59972220573	59972780747	59974318897
rate	458	461	462
requests	27493	27685	27744
status_codes.200	14997	15100	15134
status_codes.201	4999	5034	5044
status_codes.204	7497	7551	7566
success	1	1	1
throughput	458	461	462
wait	989918	896223	898586