Repeated assoc-in, assoc no longer throw NPE

[lread]

Also checked update and update-in, they were unaffected because they do not currently attempt any indentation for new items.

Note, I renamed "0.3.0" header in changelog to "Unreleased" because there is no 0.3.0 yet as far as I can tell.

Closes #19

[borkdude]

@lread I like where this is going, but perhaps we can prevent the roundtrip to fix metadata like so:

When returning a node, we can mark it as needing metadata location fixing. On an incoming node, we can then check if we need to fix locations and do it only when necessary.

[lread]

Yeah, sounds good, can do!

[borkdude]

Looks great!