Correct attribute id format (fixes #671).

[kestarumper]

This pull request fixes #671 by escaping select item names with its base64 encoding.

[codecov]

Codecov Report

Merging #672 into master will decrease coverage by 0.98%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
- Coverage   99.32%   98.34%   -0.99%     
==========================================
  Files         163      163              
  Lines        1781     1808      +27     
  Branches      551      560       +9     
==========================================
+ Hits         1769     1778       +9     
- Misses          1       10       +9     
- Partials       11       20       +9

Impacted Files	Coverage Δ
packages/uniforms-unstyled/src/SelectField.tsx	92% <33.33%> (-8%)	⬇️
packages/uniforms-bootstrap4/src/RadioField.tsx	75% <33.33%> (-25%)	⬇️
packages/uniforms-semantic/src/SelectField.tsx	92.3% <33.33%> (-7.7%)	⬇️
packages/uniforms-bootstrap3/src/SelectField.tsx	92.3% <33.33%> (-7.7%)	⬇️
packages/uniforms-material/src/SelectField.tsx	95.12% <33.33%> (-4.88%)	⬇️
packages/uniforms-unstyled/src/RadioField.tsx	77.77% <33.33%> (-22.23%)	⬇️
packages/uniforms-bootstrap3/src/RadioField.tsx	77.77% <33.33%> (-22.23%)	⬇️
packages/uniforms-semantic/src/RadioField.tsx	80% <33.33%> (-20%)	⬇️
packages/uniforms-bootstrap4/src/SelectField.tsx	92.3% <33.33%> (-7.7%)	⬇️

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[radekmie]

We are aware of the code duplication. It'll be addressed in v3.