proper header handling

[n1ru4l]

handle accept and content-type headers according to the GraphQL over HTTP specification.

[github-actions]

✅ Benchmark Results

     ✓ no_errors
     ✓ expected_result

     checks.........................: 100.00% ✓ 30802       ✗ 0    
     data_received..................: 3.5 MB  347 kB/s
     data_sent......................: 4.2 MB  420 kB/s
     http_req_blocked...............: avg=1.44µs   min=900ns   med=1.2µs   max=200.2µs p(90)=1.8µs   p(95)=2.29µs 
     http_req_connecting............: avg=9ns      min=0s      med=0s      max=143.9µs p(90)=0s      p(95)=0s     
   ✓ http_req_duration..............: avg=554.25µs min=402.7µs med=482.1µs max=15.56ms p(90)=642.2µs p(95)=734.3µs
       { expected_response:true }...: avg=554.25µs min=402.7µs med=482.1µs max=15.56ms p(90)=642.2µs p(95)=734.3µs
     http_req_failed................: 0.00%   ✓ 0           ✗ 15401
     http_req_receiving.............: avg=21.02µs  min=10.5µs  med=18.2µs  max=3.05ms  p(90)=25µs    p(95)=31.6µs 
     http_req_sending...............: avg=10.68µs  min=5.6µs   med=8.4µs   max=4.17ms  p(90)=13.3µs  p(95)=14.5µs 
     http_req_tls_handshaking.......: avg=0s       min=0s      med=0s      max=0s      p(90)=0s      p(95)=0s     
     http_req_waiting...............: avg=522.54µs min=380.3µs med=453.1µs max=15.37ms p(90)=609.5µs p(95)=702.8µs
     http_reqs......................: 15401   1539.822375/s
     iteration_duration.............: avg=643.24µs min=471.4µs med=566.9µs max=16.16ms p(90)=737.3µs p(95)=830.1µs
     iterations.....................: 15401   1539.822375/s
     vus............................: 1       min=1         max=1  
     vus_max........................: 1       min=1         max=1  

[github-actions]

The latest changes of this PR are available as canary in npm (based on the declared changesets):

graphql-helix@1.11.0-canary-fbb0037.0

[vercel]

Deployment failed with the following error:

The most recent charge for your active payment method has failed. Please update it here: https://vercel.com/teams/graphql-helix/settings/billing.