blob/s3blob: make 301 check more specific

[vangent]

Fixes #3797.

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.02%. Comparing base (f63f2a4) to head (c1e067f).

Files with missing lines	Patch %	Lines
blob/s3blob/s3blob.go	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3798      +/-   ##
==========================================
- Coverage   80.10%   80.02%   -0.08%     
==========================================
  Files         104      104              
  Lines       12315    12318       +3     
==========================================
- Hits         9865     9858       -7     
- Misses       2449     2459      +10     
  Partials        1        1              

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