Fixed parameters tuple access with unicode index (#98)

On executing context for a prepared statement which calls parameterized
functions, stored procedures, etc. HANA returns parametermetadata
containing the parameters name as id. On execution cursor.py used this
id for index access to the parameters tuple. Now an enumeration is used
to get the position of the current parameter/parametermetadata and for
access to the parameters tuple.

pyhdb/cursor.py

@@ -99,7 +99,7 @@ def __next__(self):
         if len(parameters) != len(self._params_metadata):
             raise ProgrammingError("Prepared statement parameters expected %d supplied %d." %
                                    (len(self._params_metadata), len(parameters)))
-        row_params = [self.ParamTuple(p.id, p.datatype, p.length, parameters[p.id]) for p in self._params_metadata]
+        row_params = [self.ParamTuple(p.id, p.datatype, p.length, parameters[i]) for i, p in enumerate(self._params_metadata)]
         self._iter_row_count += 1
         return row_params