Use the original arg length of wrapped CP funcs, still call the wrapper

ComputedPropertyPrototype.set analyzes the length of CP arguments to
decide how the CP function should be called. Since the wrapper (which
sets up _super) handles arbitrary arguments, the original function (at
wrappedFunction) should be inspected for .length. The wrapper, however,
should still be called when performing the set (ensuring that _super is
present).

Fixes #2890
Merge pull request #3013 from mixonic/2890

Use the original arg length of wrapped CP funcs, still call the wrapper