(#21975) Don't monkey patch Object#instance_variables

We were monkey patching Object#instance_variables to return symbols instead of
strings on ruby 1.8 which broke the use of SOAP. The patch was only used to
make it easier to do set operations when determining attributes to serialize
in zaml. This narrows the behavior of converting to symbols into the zaml code
so we don't have to override Object#instance_variables.