Fix isUpperCase and isLowerCase

[minhqdao]

The behavior of isUpperCase so far is:

'1234'.isUpperCase // true
' '.isUpperCase // true
'!'.isUpperCase // true
'\n'.isUpperCase // true
'TEST'.isUpperCase // true
'TESt'.isUpperCase // false
'HEY, YOU!'.isUpperCase // true

This is being changed to:

'1234'.isUpperCase // false
' '.isUpperCase // false
'!'.isUpperCase // false
'\n'.isUpperCase // false
'TEST'.isUpperCase // true
'TESt'.isUpperCase // false
'HEY, YOU!'.isUpperCase // true

isLowerCase is also changed, respectively.

Will update readme in #141.

[codecov]

Codecov Report

Merging #142 (21955ed) into master (6af8524) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #142   +/-   ##
=======================================
  Coverage   96.72%   96.73%           
=======================================
  Files          11       11           
  Lines         763      765    +2     
=======================================
+ Hits          738      740    +2     
  Misses         25       25           

Impacted Files	Coverage Δ
lib/src/string.dart	86.44% <100.00%> (+0.47%)	⬆️

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[minhqdao]

@passsy

[passsy]

Definitly better than before 👍