MapEncoder: Store strings in map when AddByteString is used

[prashantv]

Fixes #503.

[codecov]

Codecov Report

Merging #504 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #504   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files          37       37           
  Lines        2243     2243           
=======================================
  Hits         2180     2180           
  Misses         56       56           
  Partials        7        7

Impacted Files	Coverage Δ
zapcore/memory_encoder.go	98.68% <100%> (ø)	⬆️

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[akshayjshah]

If you also add a changelog entry, we can just cut a patch as soon as this lands.