contract-sdk/specs/token/oas2: Fix and improve self-transfer

[peternose]

No description provided.

[netlify]

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Name	Link
🔨 Latest commit	3e39831
🔍 Latest deploy log	https://app.netlify.com/projects/oasisprotocol-oasis-sdk/deploys/69ccdf4651daf10008455f3d

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.34%. Comparing base (b559f59) to head (3e39831).

Files with missing lines	Patch %	Lines
contract-sdk/specs/token/oas20/src/helpers.rs	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2450      +/-   ##
==========================================
- Coverage   51.35%   51.34%   -0.01%     
==========================================
  Files         237      237              
  Lines       17106    17111       +5     
==========================================
+ Hits         8784     8785       +1     
- Misses       8262     8266       +4     
  Partials       60       60              

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