Fix month and year picker disabled

[shu1007]

Nice to meet you. We have fixed some of the issues mentioned in the issue.(#2912 #3804 )

• YearMonthPicker and yearPicker can now select the same month/year as the specified minDate and maxDate.
• Some Unit test case added

[codecov]

Codecov Report

Merging #3888 (25575b5) into master (31e35fc) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3888      +/-   ##
==========================================
- Coverage   93.66%   93.63%   -0.03%     
==========================================
  Files          20       20              
  Lines        1373     1383      +10     
  Branches      383      388       +5     
==========================================
+ Hits         1286     1295       +9     
- Misses         25       26       +1     
  Partials       62       62              

Impacted Files	Coverage Δ
src/date_utils.js	99.15% <100.00%> (+<0.01%)	⬆️
src/index.jsx	90.06% <100.00%> (+0.22%)	⬆️
src/month.jsx	95.04% <100.00%> (ø)
src/year.jsx	92.45% <100.00%> (-1.78%)	⬇️

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[shu1007]

The code for which coverage is noted is not the one that has been corrected at this time.
https://app.codecov.io/gh/Hacker0x01/react-datepicker/pull/3888

Could you do a code review?

[martijnrusschen]

Thanks for your fix and extensive tests on this subject