Fix unhandled default params

[gandarez]

This PR fixes unhandled default params when flag/config is not set. Currently any value informed from flag/config is completely ignored for rate limit and timeout. The vipertools.FirstNonEmptyInt() function now checks if value is set and accepts zero as valid one.

Some tests have been added to ensure the right behavior and increase coverage.

Fixes #1147

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 12 lines in your changes missing coverage. Please review.

Project coverage is 62.99%. Comparing base (92addb5) to head (15f70f2).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
pkg/api/error.go	16.66%	10 Missing ⚠️
cmd/heartbeat/heartbeat.go	0.00%	2 Missing ⚠️

@@             Coverage Diff             @@
##           develop    #1154      +/-   ##
===========================================
+ Coverage    62.97%   62.99%   +0.02%     
===========================================
  Files          386      386              
  Lines        16887    16914      +27     
===========================================
+ Hits         10634    10655      +21     
- Misses        5679     5686       +7     
+ Partials       574      573       -1     

Flag	Coverage Δ
unittests	62.99% <72.72%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cmd/params/params.go	81.37% <100.00%> (+0.18%)	⬆️
pkg/api/heartbeat.go	80.00% <100.00%> (+0.22%)	⬆️
pkg/vipertools/vipertools.go	100.00% <100.00%> (ø)
cmd/heartbeat/heartbeat.go	76.67% <0.00%> (+1.48%)	⬆️
pkg/api/error.go	7.40% <16.66%> (+2.64%)	⬆️