ruby-grape / grape / 36438044714

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Pull #3015

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Answer in the default format when a negotiated format cannot render the body

On an API that pins no format, content negotiation can pick a format
the endpoint's body cannot be rendered in. A browser's Accept header
(text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8)
negotiates XML at q=0.9, and axios's (application/json, text/plain, */*)
ties at q=1 and Rack picks the first registered type, XML. The XML
formatter cannot render a String, so every endpoint returning one
answered those clients with a 500 ("cannot convert String to xml"), as
did `.xml` and `?format=xml`.

When the negotiated formatter raises InvalidFormatter, the Formatter now
renders the body in the default format, with its content type. It does
so only while the format is the client's alone: an API that pins a
format, an endpoint that set a Content-Type, or one that switched
`api_format` still answers 500, since a body that does not fit a choice
the API made is the API's error. If the default format cannot render
the body either, the 500 stands.

`api_spec.rb`'s "sets content type for xml" requested `/foo.xml` from an
endpoint returning a String, so it was checking the content type of that
500; it now requests a body XML can render.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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