Remove deepAssign, use lodash.merge to fix null fragment bug

[helfer]

Fixes #678

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change
• Add your name and email to the AUTHORS file (optional)

[helfer]

lodash.merge seems to be quite large, it brings the size to 40.8Kb, which causes tests to fail. We could bump the size for now to merge the fix, and then look for a smaller library.

[stubailo]

Sgtm