#186 - DateTime attributes not available to add as filters after WFS harvest.

Wasn't added as there was no associated filter type for this type of attribute.  Use a Date filter for DateTime attributes.
Removing unnecessary test (unnecessary because of the way we're testing the typeFromString(...) method).
Merge pull request #219 from aodn/issue-186

#186 - DateTime attributes not available to add as filters after WFS har...