Add GetHeader() method

[dtitov]

No description provided.

[codecov]

Codecov Report

Merging #1 into master will increase coverage by 0.24%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   65.37%   65.62%   +0.24%     
==========================================
  Files           6        6              
  Lines         803      803              
==========================================
+ Hits          525      527       +2     
+ Misses        161      160       -1     
+ Partials      117      116       -1     

Flag	Coverage Δ
#unittests	65.62% <95.34%> (+0.24%)	⬆️

Impacted Files	Coverage Δ
streaming/in.go	68.51% <77.77%> (+0.09%)	⬆️
keys/keys.go	66.52% <100.00%> (+0.26%)	⬆️
model/headers/headers.go	56.43% <100.00%> (+0.28%)	⬆️
streaming/out.go	78.78% <100.00%> (ø)

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