Mention forcing initial rerender

[RoryGudka]

Lines 413 to 415 of quill.mention.csj.js has the value method of the MentionBlot as follows:

function value(domNode) {
return domNode.dataset;
}

However, this returns a DOMStringMap, which is considered different from the object format storable in JSON, and it triggers a rerender of the editor. While harmless in simple editors, this can cause significant issues as the editor gains complexity. Switching it to the following fixed a number of problems for the project I'm working on:

function value(domNode) {
return { ...domNode.dataset };
}

[laikee99]

well work, I've do the same thing.

[MadSpindel]

Could you submit a PR?