Increase test coverage

[icanzilb]

Currently the test coverage fluctuates but as soon as the base set of features is fixed in the way they should work, we should add more tests around:

• making sure there are tests for roundtrip of all supported types of values (e.g. Int, Double, Float, etc.)
• testing the error scenarios (e.g. invalid URLs found in the markdown, etc.)
• research testing more edge-cases than we have now (i.e. for example currently there are unit tests for nil lists, empty lists, lists containing a single nil value, etc. so in similar fashion we should test for more non-trivial use cases)