ExecuteQuery() function should always return a dict of lists

[sduenas]

The behavior of ExecuteQuery() is not always the same. When the result of the query is just one element, the function returns a dict containing that element:

{'is_response_of': None, 'message_ID': '<[email protected]>'}

On the other hand, when the result is composed by many elements, it returns a dict of lists:

{'is_response_of': [None, '<[email protected]>'], 
'message_ID': ['<[email protected]>', '<[email protected]>']}

But, when the result is empty, ExecuteQuery() returns a dict of empty lists:

{'is_response_of': [], 'message_ID': []}

This function should always return the same type of objects to avoid errors accessing the results of the query.