fix datetime and number format fallbacking

[kazupon]

closes #852

[codecov-io]

Codecov Report

Merging #858 into v8.x will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             v8.x     #858      +/-   ##
==========================================
+ Coverage   96.41%   96.45%   +0.04%     
==========================================
  Files          10       10              
  Lines         864      874      +10     
==========================================
+ Hits          833      843      +10     
  Misses         31       31              

Impacted Files	Coverage Δ
src/components/number.js	100.00% <100.00%> (ø)
src/index.js	97.93% <100.00%> (+0.04%)	⬆️
src/util.js	98.33% <100.00%> (ø)

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