Serialize DocumentReadyState

[Sebbs128]

Types of Changes

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• [X] I have read the CONTRIBUTING document
• [X] My code follows the code style of this project

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• [X] Bug fix (non-breaking change which fixes an issue, please reference the issue id)
• [ ] New feature (non-breaking change which adds functionality, make sure to open an associated issue first)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [X] I have added tests to cover my changes
• [X] All new and existing tests passed

Description

Serializes the DocumentReadyState enum for document.readyState based on the DomNameAttribute.OfficialName value if available. Serializing other enum types has been left open, as too many tests/intended functionality breaks otherwise.

The serialized values are only available when AngleSharp v1.0.2 or greater is installed, otherwise previous behaviour is preserved.

Addresses #86

[CLAassistant]

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