Fix for the issue filter_post_data_parameters not working with aiohttp #398

[vprakashplanview]

A test_replace_post_data_parameters_with_body_type_as_dict is added under /vcrpy/tests/unit/test_filters.py

[scop]

I implemented a fix for this before noticing this PR. However I don't think the implementation here is correct -- I believe it would not work appropriately when filter_post_data_parameters or remove_post_data_parameters delegates to it, nor it behaves as the API docs say with regards to replacements possibly being callables: https://vcrpy.readthedocs.io/en/latest/advanced.html#advanced-use-of-filter-headers-filter-query-parameters-and-filter-post-data-parameters

My version of the fix in #582 should behave appropriately with regards to these issues, and has some more unit test to showcase it.