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Added support for the ``Content-Range`` HTTP header in the GET endpoint of
``PiccoloCRUD``. This means the API client can fetch the number of available
rows, without doing a separate API call to the ``count`` endpoint.
.. code-block::
GET /?__range_header=true
If the page size is 10, then the response header then looks something like:
.. code-block::
Content-Range: movie 0-9/100
The feature was created to make Piccolo APIs work better with front ends like
`React Admin <https://marmelab.com/react-admin/>`_.
Thanks to trondhindenes for adding this feature, and sinisaos for help
reviewing.
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