Add a new argument to `get_all_records` to provide the list of expected
headers.

The given expected headers must:
- be unique
- be part of the complete headers list
- must not contain extra headers

This will provide a way for users to use this method and still
have *some* duplicated headers that are not relevant to pull.

This will ensure the columns that matters have unique headers.

Closes #1007

Improved test coverage, make sure the logic is and remains.
Add note to users to expect all headers to be returned even if only a
subset is provided by `expected_headers`.
Updated test cassette on the way.
Clean old comment in tests when preparing sheet data.