fix document_ref.set function for SetOption working

[hi-jin]

Hi, I'm a mobile application developer using your localstore library!
This is a pull request for better functionality.

Before this commit

document_ref.set(
    {
        "data": "something",
    },
    SetOption(merge: true),
);

doesn't work.
Because _data doesn't have the document data.
So empty Map + {"data": "something"} just go into the existing doc. (In fact, this is merge: false)

After this commit

SetOption works properly.

Related Issue

Closes #10