fix refreshable credentials injection

[thehesiod]

fixes issue #1313

[codecov]

Codecov Report

Attention: Patch coverage is 56.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 90.53%. Comparing base (1515727) to head (3ee8285).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
aiobotocore/credentials.py	45.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1314      +/-   ##
==========================================
- Coverage   90.76%   90.53%   -0.23%     
==========================================
  Files          68       68              
  Lines        6542     6564      +22     
==========================================
+ Hits         5938     5943       +5     
- Misses        604      621      +17     

Flag	Coverage Δ
unittests	90.53% <56.00%> (-0.23%)	⬇️

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[thehesiod]

the base class impls may never get called, just there for correctness

[jakob-keller]

Nice and clean. Thanks for the fix!