Make markers created by Writer#insert() affect data.

[ma2ciek]

Suggested merge commit message (convention)

Fix: Made markers created by Writer#insert() affect data. Closes ckeditor/ckeditor5#4438.

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Additional information

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[coveralls]

Coverage remained the same at 100.0% when pulling c478a3e on t/1583 into f88c918 on master.

[oskarwrobel]

Travis says:

Chrome 70.0.3538 (Linux 0.0.0) Writer insert() should transfer markers from given DocumentFragment FAILED
	AssertionError: expected undefined to equal true
	    at Context.it (webpack:///./tests/model/writer.js?:331:38)