Fix segfault with TestSetGet.

TestSetGet was segfaulting since vtkWebGLExporter was not initializing a char*.
Fixed that by using a std::string.

Change-Id: Iacb6c5c6d2f9170145e400e271a2d92dcdb8e81f
Merge topic 'fix_segfault' into master

b80e196c Fix segfault with TestSetGet.