Serialize DocumentReadyState

[Sebbs128]

Types of Changes

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• Bug fix (non-breaking change which fixes an issue, please reference the issue id)
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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]

All committers have signed the CLA.

[FlorianRappl]

Thanks!