dispatch proper values in Python write plugin

dispatch proper values in Python write plugin

Fixes the Python write callback so the appropriate value is dispatched to
Python. Previously, the code only looked at the first element of a data set
to determine which value type (GAUGE, COUNTER, etc) to dispatch. If your data
set consisted of multiple values of different types, then the Python write
plugin was receiving bad values for the elements at position n > 0 whose type
was not the same as that at position 0.