Fix bug with unique adds and delete_existing

[mwichmann]

AppendUnique and PrependUnique, when called with delete_existing true, had a small logic flaw: it might remove an existing value if the value to be added s a scalar (string, most likely) and there was a substring match. The code needs to do an equality test, not a membership test.

Docstrings and unit tests updated, plus dropped a duplicate definition of reserved_variables in the test.

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt (and read the README.rst)
• [X] I have updated the appropriate documentation