fix(strategy): move type handling for storage to property

[nijel]

Avoid putting the burden further down, rather wrap this (unusual) case directly in the class.

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.00%. Comparing base (377b9e4) to head (2de4f22).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/strategy.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1502      +/-   ##
==========================================
+ Coverage   78.98%   79.00%   +0.01%     
==========================================
  Files         354      354              
  Lines       11516    11510       -6     
  Branches      530      526       -4     
==========================================
- Hits         9096     9093       -3     
+ Misses       2238     2236       -2     
+ Partials      182      181       -1     

Flag	Coverage Δ
unittests	79.00% <96.66%> (+0.01%)	⬆️

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