feat: allow headers to accept fn

[anshumanv]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Headers can now accept fn

Breaking Changes

no

Additional Info

Fix #890 fix #892

[codecov]

Codecov Report

Merging #897 (9116e75) into master (594dddc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #897   +/-   ##
=======================================
  Coverage   98.58%   98.59%           
=======================================
  Files          10       10           
  Lines         282      284    +2     
  Branches       99      100    +1     
=======================================
+ Hits          278      280    +2     
  Misses          4        4           

Impacted Files	Coverage Δ
src/middleware.js	100.00% <100.00%> (ø)

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[alexander-akait]

Let's add test and when send the same PR to dev server, because we have the headers option on dev server level too

[anshumanv]

wip 👍

[snitin315]

Thanks!