add string dataset condition for data type conversion on export

[stephprince]

Motivation

This issue came up in NeurodataWithoutBorders/pynwb#1973 during export of an NWBFile. During export, the ObjectMapper attempts to change the dtype of a StrDataset, which causes the data to be converted from a StrDataset to an AstypeWrapper h5py object. This conversion causes an error for the DatasetBuilder.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.12%. Comparing base (ea6504f) to head (5c56d4d).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1205   +/-   ##
=======================================
  Coverage   89.12%   89.12%           
=======================================
  Files          45       45           
  Lines        9945     9947    +2     
  Branches     2826     2827    +1     
=======================================
+ Hits         8863     8865    +2     
  Misses        762      762           
  Partials      320      320           

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[rly]

Thank you @stephprince !