added null checks to NewCarbonInterval

[ashwek]

package main

import "github.com/uniplaces/carbon"

func main() {
	carbon.NewCarbonInterval(nil, nil)
}

This simple code would panic because there are no null checks in NewCarbonInterval.

carbon/carboninterval.go

Line 16 in 1c77b65

return nil, errors.New("The end date must be greater than the start date.")

According to Go Code Review Comments

Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with punctuation, since they are usually printed following other context.

[rntdrts]

Hi @ashwek, thank you for contributing. Can you please add tests as well?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ce063dd) 89.11% compared to head (981f9df) 89.45%.
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
+ Coverage   89.11%   89.45%   +0.34%     
==========================================
  Files           3        4       +1     
  Lines         634      645      +11     
==========================================
+ Hits          565      577      +12     
  Misses         44       44              
+ Partials       25       24       -1     

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[ashwek]

@rntdrts added test cases. Can you please review the changes