add gas price=0 unit test, comments

[noot]

Closes: #XXX

Description

follow up to #515 , #516

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[fedekunze]

thanks!

[codecov]

Codecov Report

Merging #528 into development will increase coverage by 0.17%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #528      +/-   ##
===============================================
+ Coverage        70.60%   70.77%   +0.17%     
===============================================
  Files               41       41              
  Lines             2320     2320              
===============================================
+ Hits              1638     1642       +4     
+ Misses             538      536       -2     
+ Partials           144      142       -2     

Impacted Files	Coverage Δ
x/evm/handler.go	91.37% <ø> (ø)
x/evm/types/msg.go	67.39% <0.00%> (+2.89%)	⬆️

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