Incorrect $format passed to the oembed_endpoint_url filter

[swissspidy]

In get_oembed_endpoint_url(). $format is set to false when JSON is passed as the format.

This is done so that the JSON format (which is the default) isn't unnecessarily appended to the URL.

However, when using the oembed_endpoint_url filter, $format is always false in that case. The filter docs state a string is passed, so this is unexpected behaviour.

The add_query_arg() part should be changed to read something like format' => ( 'json' === $format ) ? false : $format,

[danhgilmore]

Patch to check if format is JSON in add_query_arg()

[SergeyBiryukov]

In 38321:

Embeds: In get_oembed_endpoint_url(), avoid inadvertent stomping of the $format parameter passed to oembed_endpoint_url filter.

Props danhgilmore.
Fixes #37751.