Fix ValueBuffer#Copy method

[asdine]

This PR fixes a bug that made the ValueBuffer#Copy method to append nested arrays and nested documents at the end of the array.

Before:

genji> create table foo (a.1 int primary key);
genji> insert into foo (a) VALUES ([1, 2, [3, 4]]);
genji> select * from foo;
{
  "a": [
    1,
    2,
    [
      3,
      4
    ],
    [
      3,
      4
    ]
  ]
}

After:

genji> create table foo (a.1 int primary key);
genji> insert into foo (a) VALUES ([1, 2, [3, 4]]);
genji> select * from foo;
{
  "a": [
    1,
    2,
    [
      3,
      4
    ]
  ]
}

Thank you @tzzed for spotting this bug!