Use data-type specific conversion functions also in plpy.execute

In PLy_spi_execute_plan, use the data-type specific Python-to-PostgreSQL
conversion function instead of passing everything through InputFunctionCall
as a string.  The equivalent fix was already done months ago for function
parameters and return values, but this other gateway between Python and
PostgreSQL was apparently forgotten.  As a result, data types that need
special treatment, such as bytea, would misbehave when used with
plpy.execute.