json: Fix float32 encoding and add test

[manjari25]

This commit fixes incorrect float32 encoding. A test
case is also added.

Refs:

• Bug: Incorrect encoding of float32 numbers #1002
• Internal ticket: GO-860

[codecov]

Codecov Report

Merging #1003 (98d4f19) into master (914c4ff) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1003      +/-   ##
==========================================
+ Coverage   98.10%   98.20%   +0.09%     
==========================================
  Files          46       46              
  Lines        2057     2058       +1     
==========================================
+ Hits         2018     2021       +3     
+ Misses         30       29       -1     
+ Partials        9        8       -1     

Impacted Files	Coverage Δ
zapcore/json_encoder.go	100.00% <100.00%> (ø)
zapcore/sampler.go	100.00% <0.00%> (+3.77%)	⬆️

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[sywhang]

LGTM