[6.0]optimize header value code

[NathanFreeman]

// without this patch benchmarks
wrk -t4 -c500 -d30s -s script.lua http://127.0.0.1/
Running 30s test @ http://127.0.0.1/
  4 threads and 500 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.51ms    1.92ms  23.82ms   72.97%
    Req/Sec    25.95k     1.68k   40.07k    74.08%
  3101765 requests in 30.08s, 485.12MB read
Requests/sec: 103102.72
Transfer/sec:     16.13MB

// with this patch benchmarks
wrk -t4 -c500 -d30s -s script.lua http://127.0.0.1/
Running 30s test @ http://127.0.0.1/
  4 threads and 500 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.41ms    1.92ms  27.18ms   72.86%
    Req/Sec    26.43k     1.66k   31.30k    72.49%
  3162096 requests in 30.08s, 494.56MB read
Requests/sec: 105121.75
Transfer/sec:     16.44MB

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.85%. Comparing base (3cdb863) to head (17c428b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5605      +/-   ##
==========================================
- Coverage   70.93%   70.85%   -0.09%     
==========================================
  Files         109      109              
  Lines       16757    16772      +15     
  Branches     3061     3067       +6     
==========================================
- Hits        11887    11883       -4     
- Misses       4814     4833      +19     
  Partials       56       56              

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