Unit tests & CI checks

[ddymko]

Description

• Adding gofmt and golint checks to PRs
• Added missing unit tests and more tests for govultr_test.go
• Fixed typo in URI for detach private networks in instances (bug)

Related Issues

n/a

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Merging #122 (b7267c4) into master (e1ebd44) will increase coverage by 5.86%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   49.14%   55.00%   +5.86%     
==========================================
  Files          23       23              
  Lines        1168     1169       +1     
==========================================
+ Hits          574      643      +69     
+ Misses        348      264      -84     
- Partials      246      262      +16     

Impacted Files	Coverage Δ
user.go	50.00% <0.00%> (-2.95%)	⬇️
govultr.go	90.58% <100.00%> (+10.35%)	⬆️
instance.go	55.84% <100.00%> (+14.71%)	⬆️
bare_metal_server.go	52.94% <0.00%> (+8.82%)	⬆️
load_balancer.go	50.72% <0.00%> (+10.14%)	⬆️
network.go	51.35% <0.00%> (+21.62%)	⬆️

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