http patch bugfix: make unknown header names case insensitive

[jbr]

closes #311

also relaxes some [Headers] lifetime requirements, closing #310

[codecov]

Codecov Report

Merging #312 (43fb560) into main (5a52f44) will increase coverage by 0.26%.
The diff coverage is 58.10%.

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
+ Coverage   42.31%   42.58%   +0.26%     
==========================================
  Files         139      141       +2     
  Lines        5494     5507      +13     
==========================================
+ Hits         2325     2345      +20     
+ Misses       3169     3162       -7     

Impacted Files	Coverage Δ
http/src/headers/header_value.rs	39.62% <ø> (ø)
http/src/headers/unknown_header_name.rs	47.36% <47.36%> (ø)
http/src/headers/header_name.rs	48.57% <48.57%> (ø)
http/src/headers/known_header_name.rs	11.68% <50.00%> (ø)
http/src/headers.rs	67.40% <92.85%> (+3.49%)	⬆️
http/src/headers/header_values.rs	57.77% <100.00%> (ø)

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