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.

src/python.c

@@ -345,26 +345,26 @@ static int cpy_write_callback(const data_set_t *ds, const value_list_t *value_li
 			CPY_RETURN_FROM_THREADS 0;
 		}
 		for (i = 0; i < value_list->values_len; ++i) {
-			if (ds->ds->type == DS_TYPE_COUNTER) {
+			if (ds->ds[i].type == DS_TYPE_COUNTER) {
 				if ((long) value_list->values[i].counter == value_list->values[i].counter)
 					PyList_SetItem(list, i, PyInt_FromLong(value_list->values[i].counter));
 				else
 					PyList_SetItem(list, i, PyLong_FromUnsignedLongLong(value_list->values[i].counter));
-			} else if (ds->ds->type == DS_TYPE_GAUGE) {
+			} else if (ds->ds[i].type == DS_TYPE_GAUGE) {
 				PyList_SetItem(list, i, PyFloat_FromDouble(value_list->values[i].gauge));
-			} else if (ds->ds->type == DS_TYPE_DERIVE) {
+			} else if (ds->ds[i].type == DS_TYPE_DERIVE) {
 				if ((long) value_list->values[i].derive == value_list->values[i].derive)
 					PyList_SetItem(list, i, PyInt_FromLong(value_list->values[i].derive));
 				else
 					PyList_SetItem(list, i, PyLong_FromLongLong(value_list->values[i].derive));
-			} else if (ds->ds->type == DS_TYPE_ABSOLUTE) {
+			} else if (ds->ds[i].type == DS_TYPE_ABSOLUTE) {
 				if ((long) value_list->values[i].absolute == value_list->values[i].absolute)
 					PyList_SetItem(list, i, PyInt_FromLong(value_list->values[i].absolute));
 				else
 					PyList_SetItem(list, i, PyLong_FromUnsignedLongLong(value_list->values[i].absolute));
 			} else {
 				Py_BEGIN_ALLOW_THREADS
-				ERROR("cpy_write_callback: Unknown value type %d.", ds->ds->type);
+				ERROR("cpy_write_callback: Unknown value type %d.", ds->ds[i].type);
 				Py_END_ALLOW_THREADS
 				Py_DECREF(list);
 				CPY_RETURN_FROM_THREADS 0;