don't skip explicit str tags when emitting

[Herringway]

Fixes an issue uncovered by #316

[codecov]

Codecov Report

Merging #317 (c406a81) into master (a914d23) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #317   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files          29       29           
  Lines        4505     4505           
=======================================
  Hits         4286     4286           
  Misses        219      219           

Files Changed	Coverage Δ
source/dyaml/emitter.d	95.39% <100.00%> (ø)

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