Improved conversion in disallowed context

[oskarwrobel]

Suggested merge commit message (convention)

Internal: Improved conversion in disallowed context. Closes ckeditor/ckeditor5#3001.

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Needs:

• Fix: Added missing parse context in DataController#set(). ckeditor5-engine#1280
• Internal: Set correct modelCursor in v->m docFrag converter. ckeditor5-engine#1279

[coveralls]

Coverage decreased (-0.2%) to 99.798% when pulling 0939429 on t/93 into d1fae4b on master.