Fix error with setting read_io to same obj twice

[rly]

Motivation

Fix #914.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (ff30e12) 88.32% compared to head (0da71eb) 88.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #915   +/-   ##
=======================================
  Coverage   88.32%   88.33%           
=======================================
  Files          45       45           
  Lines        9280     9283    +3     
  Branches     2651     2651           
=======================================
+ Hits         8197     8200    +3     
  Misses        765      765           
  Partials      318      318           

Files Changed	Coverage Δ
src/hdmf/container.py	91.54% <100.00%> (+0.03%)	⬆️

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