admin: fix ModelAdmin: only disallow POST

[blueyed]

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[codecov]

Codecov Report

Merging #106 into next will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             next     #106     +/-   ##
=========================================
+ Coverage   98.86%   98.97%   +0.1%     
=========================================
  Files          34       34             
  Lines        1935     1942      +7     
  Branches      173      173             
=========================================
+ Hits         1913     1922      +9     
+ Misses         13       12      -1     
+ Partials        9        8      -1

Flag	Coverage Δ
#py27dj111	98.97% <100%> (+0.1%)	⬆️
#py36dj20psql	98.97% <100%> (+0.1%)	⬆️

Impacted Files	Coverage Δ
pinax/stripe/admin.py	98.98% <100%> (+1.08%)	⬆️

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