fix: avoid duplicate iframe children cloning

[andyearnshaw]

Description

Motivation and Context

Fixes #543.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Enhancement (changes that improvement of current feature or performance)
• [ ] Refactoring (changes that neither fixes a bug nor adds a feature)
• [x] Test Case (changes that add missing tests or correct existing tests)
• [ ] Code style optimization (changes that do not affect the meaning of the code)
• [ ] Docs (changes that only update documentation)
• [ ] Chore (changes that don't modify src or test files)

Self Check before Merge

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.50%. Comparing base (d9b2fcf) to head (e7d37a8). Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #551   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          10       10           
  Lines         612      612           
  Branches      150      150           
=======================================
  Hits          407      407           
  Misses        144      144           
  Partials       61       61           

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[ruediste]

merged to https://github.com/ruediste/html-to-image