Clean up the flattening separator feature

[juanjoDiaz]

Clean up a bit after #314.

• Improves function signature bringing it back to the original approach.
• Add missing tests.
• Fix a typo in the old tests names.

Also, next time we merge a change that modifies the functions signatures, please create a task so I update the TypeScript types.

[coveralls]

Coverage remained the same at 100.0% when pulling b325788 on juanjoDiaz:flattening_separator_cleanup into 24d7654 on zemirco:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b325788 on juanjoDiaz:flattening_separator_cleanup into 24d7654 on zemirco:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b325788 on juanjoDiaz:flattening_separator_cleanup into 24d7654 on zemirco:master.

[knownasilya]

Good point about the types. Want to rewrite this in TS so we have types automatically? 😜