Update document-cloner.ts

[bigbone]

the method of getComputedStyle.content maybe 'normal'.

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Summary

This PR fixes/implements the following bugs/features

• [ ] Bug 1
• [ ] Bug 2
• [ ] Feature 1
• [ ] Feature 2
• [ ] Breaking changes

Explain the motivation for making this change. What existing problem does the pull request solve?

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Demonstrate how the issue/feature can be replicated. For most cases, simply adding an appropriate html/css template into the reftests should be sufficient. Please see other tests there for reference.

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Closing issues

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[Sharcoux]

Can be tested in @cantoo/html2canvas