Add a final line-ending if multi-line input string

[gdiepen]

Without this final line-ending, reordering steps will fail because tables will be added directly to the previous value (see issue #381)

when creating the string from the Container again, we remove any final line-ending if we added it initially to ensure we do not modify the original behavior

This modification only fails on two unit tests. However, I do not know enough about the TOML specifiation to see why this is a problem. Is there anybody that could help out and point me to a direction on how I can fix these final two unit tests?

[gdiepen]

Checked the two unit tests that are failing, one is doing a parse, copy, as_string of [foo]\nbar=1\n and after the three steps, it is now missing the final newline.

Would this not need to actually have the trailing newline?

The other unit tests that now fails is parsing the value:
tomlkit.array("""[\n"alpha",\n"omega"\n]""")

Not sure how I would need to fix this.

An alternative to my current approach of adding a final new-line to the input string would be to fix this when reordering the items in an array instead.