Fix lexer allow T-prefix on local dates

[AllenX2018]

The dateRegex matched localDate with T, we should remove it.

Fix #442

[codecov]

Codecov Report

Merging #446 into master will decrease coverage by 0.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
- Coverage   93.91%   93.32%   -0.59%     
==========================================
  Files          10       10              
  Lines        2448     2158     -290     
==========================================
- Hits         2299     2014     -285     
+ Misses        108      104       -4     
+ Partials       41       40       -1     

Impacted Files	Coverage Δ
lexer.go	98.68% <100.00%> (-0.15%)	⬇️
toml.go	73.12% <0.00%> (-2.90%)	⬇️
tomltree_write.go	94.61% <0.00%> (-0.54%)	⬇️
marshal.go	95.86% <0.00%> (-0.47%)	⬇️
parser.go	90.66% <0.00%> (-0.11%)	⬇️
token.go	100.00% <0.00%> (ø)
position.go	100.00% <0.00%> (ø)
localtime.go	100.00% <0.00%> (ø)
keysparsing.go	100.00% <0.00%> (ø)
... and 1 more

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[pelletier]

Thanks! Not sure why codecov is detecting so much "coverage change"...