Return tagged items that are assigned to a heading

[AlexanderWillner]

To Reproduce Steps to reproduce the behavior: E.g. things.tasks(tag="Errand", project="3x1QqJqfvZyhtw8NSdnZqG")

Expected behavior Return a task that is assigned to a heading in that project and tagged with Errand

Additional context Works fine with tasks not assigned to a heading and fine when not filtering by a tag.

[AlexanderWillner]

@mikez you've probably an opinion on how to fix this in a better way ;) https://github.com/AlexanderWillner/things.py/commit/c95bab475b61bf54288ef129de0e7f392cd015b4