COMP: Fix type casting build warning.

This patch fixes the build warning by a gcc compiler in the dashboard over
some implicit type-casting being performed in the
itk::SigmoidTransferFunctionTest. Compiles fine under Windows, Visual
Studio 2010.

Change-Id: Iec248a3683461babc9cf6a5b55c51fd533db339c
Merge topic 'SigmoidFix'

455a2918 COMP: Fix type casting build warning.