Preservable Keys: Extract into method

[GaryJones]

• Use consistent terminology in code: "preserved" (which is what the filter is called), rather than "protected".
• Moves the decoupling of the preservable keys and values that are present in a URL, to its own method. Moving it away from a static method / into a separate class is beyond the scope of this PR, but is recognised as an issue.
• Adds documentation to the filter.
• Includes comprehensive unit test for the new method, which in turns helps define the behaviour for edge cases of different URLs and returned filter values.