Modify the behavior when execution date is Sunday

[dtan4]

If Xronor is executed at Sunday, generated result is difference from the expected one.

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #21     +/-   ##
=========================================
+ Coverage    96.4%   96.71%   +0.3%     
=========================================
  Files          31       31             
  Lines         891      912     +21     
=========================================
+ Hits          859      882     +23     
+ Misses         32       30      -2

Impacted Files	Coverage Δ
spec/lib/xronor/dsl/schedule_converter_spec.rb	100% <100%> (ø)	⬆️
lib/xronor/dsl/schedule_converter.rb	100% <100%> (+1.51%)	⬆️
spec/lib/xronor/parser_spec.rb	100% <0%> (+4.54%)	⬆️

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