Responses

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- Added `assert_call_count` to improve ergonomics around ensuring a mock was called.
- Fix incorrect handling of paths with query strings.
- Add Python 3.9 support to CI matrix.

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- Retag of 0.10.13

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- Improved README examples.
- Improved handling of unicode bodies. The inferred content-type for unicode
bodies is now `text/plain; charset=utf-8`.
- Streamlined querysting matching code.

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- Fixed incorrect content-type in `add_callback()` when headers are provided as a list of tuples.

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- Fixed invalid README formatted.
- Fixed string formatting in error message.

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- Added Python 3.8 support
- Remove Python 3.4 from test suite matrix.
- The `response.request` object now has a `params` attribute that contains the query string parameters from the request that was captured.
- `add_passthru` now supports `re` pattern objects to match URLs.
- ConnectionErrors raised by responses now include more details on the request that was attempted and the mocks registered.