While we might never be able to have 100% of our code strict, we can at
the very least do it for all of our tests. This ensures that our tests
are using our API with the types as intended by the test author, and not
silently be cast to what our API requires.
Fix#2931. If surface charts are written with c:grouping tag, Excel will treat them as corrupt. Also, some 3D-ish Xml tags are required when 2D surface charts are written, else they will be treated as 3D. Also eliminate a duplicate line identified by #2932.
Adrien Crivelli
oleibman