fix(dayjs.add): support daylight saving time

[farnabaz]

It's not safe to add 24 hours to manipulate date, There are 2 days of the year that do not have 24 hours
In order to get rid of this issue its good to use build-in setDate
See: https://en.wikipedia.org/wiki/Daylight_saving_time

[codecov-io]

Codecov Report

Merging #319 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #319   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          37     37           
  Lines         403    407    +4     
  Branches       53     55    +2     
=====================================
+ Hits          403    407    +4

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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[prantlf]

I wonder, what is the expected behaviour, if you add one day. The day, when the daylight-saving change occurs, is shorter or longer. Do you want to just flip the day number, or do you want to consider the right hours? You may need to wait for the "UTC mode" (#326), which would "only flip the day number". Day.js is not a "date only" type. There is a time part in the instance too.

This is what happened with Day.js Node.js in Central Europe this year. The day had 23 hours and both adding 1 day and 24 hours ended well:

> original = dayjs('2018-03-25')
'2018-03-24T23:00:00.000Z'
> original.toDate().toLocaleString()
'2018-3-25 00:00:00'
> tomorrow = original.add(1, 'day')
'2018-03-25T22:00:00.000Z'
> tomorrow.toDate().toLocaleString()
'2018-3-26 00:00:00'
> tomorrow24 = original.add(24, 'hour')
'2018-03-25T23:00:00.000Z'
> tomorrow24.toDate().toLocaleString()
'2018-3-26 01:00:00'

And just one day later, everything was simpler:

> original = dayjs('2018-03-26')
'2018-03-25T22:00:00.000Z'
> original.toDate().toLocaleString()
'2018-3-26 00:00:00'
> tomorrow = original.add(1, 'day')
'2018-03-26T22:00:00.000Z'
> tomorrow.toDate().toLocaleString()
'2018-3-27 00:00:00'
> tomorrow24 = original.add(24, 'hour')
'2018-03-26T22:00:00.000Z'
> tomorrow24.toDate().toLocaleString()
'2018-3-27 00:00:00'

[iamkun]

🎉 This PR is included in version 1.7.7 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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