Fix API link header

[lunny]

I also added test for link header generation and will fix #7296

[codecov-io]

Codecov Report

Merging #7298 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7298      +/-   ##
==========================================
+ Coverage   41.15%   41.21%   +0.05%     
==========================================
  Files         464      464              
  Lines       62763    62788      +25     
==========================================
+ Hits        25833    25878      +45     
+ Misses      33537    33520      -17     
+ Partials     3393     3390       -3

Impacted Files	Coverage Δ
modules/context/api.go	74.78% <100%> (+19.47%)	⬆️
routers/repo/view.go	43.25% <0%> (+1.01%)	⬆️
modules/log/event.go	65.64% <0%> (+1.02%)	⬆️
models/unit.go	67.56% <0%> (+5.4%)	⬆️

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