Fix nested sum evaluation.

Fix the evaluation of nested sums that contain a KroneckerDelta.
Also fix the evaluation of sums that contain a Piecewise, which
is the return value of deltasummation in most cases.

Fixes issue 3924: Evaluation of nested sums.
http://code.google.com/p/sympy/issues/detail?id=3924
Merge pull request #2245 from jrioux/concrete

Fix nested sum evaluation.