fix: handle whitespace in HTTP Accept header

[aloisklink]

Currently, using something like Accept: application/my-custom-type,application/json;q=0.9 correctly returns errors as JSON. However, once you add a space to it, e.g. like Accept: application/my-custom-type, application/json;q=0.9, any errors messages returned use HTML instead of JSON.

The HTTP Accept: header, according to RFC 9110

According to RFC 9110, in the HTTP Accept header, there can be OWS (optional whitespace, e.g. zero to unlimited SP (space) or HTAB (horizontal tab)) between the ; and , characters.

If you go to RFC 9110 § 12.5.1 Accept ¶ 3:

  Accept = #( media-range [ weight ] )

  media-range    = ( "*/*"
                     / ( type "/" "*" )
                     / ( type "/" subtype )
                   ) parameters

Where #() is defined as a list, see RFC 9110 § 5.6.1.2 ¶ 2

  #element => [ element ] *( OWS "," OWS [ element ] )

and where OWS is defined in RFC 9110 § 5.6.3 Whitespace ¶ 7 as:

  OWS            = *( SP / HTAB )
                 ; optional whitespace

and where weight is defined as RFC 9110 § 12.4.2 Quality Values ¶ 3 as:

  weight = OWS ";" OWS "q=" qvalue
  qvalue = ( "0" [ "." 0*3DIGIT ] )
         / ( "1" [ "." 0*3("0") ] )

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