Sets binary reader buffers to null on close().

[tgregg]

Issue #, if available:
Closes #678

Description of changes:
This allows these objects to be garbage collected even if the user holds onto the IonReader reference after calling close().

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ebf8f4d) 67.25% compared to head (5fbecf6) 67.24%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #694      +/-   ##
============================================
- Coverage     67.25%   67.24%   -0.01%     
+ Complexity     5486     5482       -4     
============================================
  Files           159      159              
  Lines         23016    23019       +3     
  Branches       4124     4124              
============================================
  Hits          15479    15479              
- Misses         6255     6256       +1     
- Partials       1282     1284       +2     

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[bousselham-mhidi]

Do you know when this PR will be merged please ?