[bugfix] Fix NegotiateAccept with multi accept

There's a bug in Gin's NegotiateFormat that doesn't handle the presence
of multilpe accept headers. This lifts the code from the PR @tsmethurst
sent a year ago to Gin into our codebase to fix the issue.

internal/api/util/negotiate_test.go

@@ -0,0 +1,65 @@
+package util
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+)
+
+type testMIMES []MIME
+
+func (tm testMIMES) String(t *testing.T) string {
+	t.Helper()
+
+	res := tm.StringS(t)
+	return strings.Join(res, ",")
+}
+
+func (tm testMIMES) StringS(t *testing.T) []string {
+	t.Helper()
+
+	res := make([]string, 0, len(tm))
+	for _, m := range tm {
+		res = append(res, string(m))
+	}
+	return res
+}
+
+func TestNegotiateFormat(t *testing.T) {
+	tests := []struct {
+		incoming []string
+		offered  testMIMES
+		format   string
+	}{
+		{incoming: testMIMES{AppJSON}.StringS(t), offered: testMIMES{AppJRDJSON, AppJSON}, format: "application/json"},
+		{incoming: testMIMES{AppJRDJSON}.StringS(t), offered: testMIMES{AppJRDJSON, AppJSON}, format: "application/jrd+json"},
+		{incoming: testMIMES{AppJRDJSON, AppJSON}.StringS(t), offered: testMIMES{AppJRDJSON}, format: "application/jrd+json"},
+		{incoming: testMIMES{AppJRDJSON, AppJSON}.StringS(t), offered: testMIMES{AppJSON}, format: "application/json"},
+		{incoming: testMIMES{"text/html,application/xhtml+xml,application/xml;q=0.9;q=0.8"}.StringS(t), offered: testMIMES{AppJSON, AppXML}, format: "application/xml"},
+		{incoming: testMIMES{"text/html,application/xhtml+xml,application/xml;q=0.9;q=0.8"}.StringS(t), offered: testMIMES{TextHTML, AppXML}, format: "text/html"},
+	}
+
+	for _, tt := range tests {
+		name := "incoming:" + strings.Join(tt.incoming, ",") + " offered:" + tt.offered.String(t)
+		t.Run(name, func(t *testing.T) {
+			tt := tt
+			t.Parallel()
+
+			c, _ := gin.CreateTestContext(httptest.NewRecorder())
+			c.Request = &http.Request{
+				Header: make(http.Header),
+			}
+			for _, header := range tt.incoming {
+				c.Request.Header.Add("accept", header)
+			}
+
+			format := NegotiateFormat(c, tt.offered.StringS(t)...)
+			if tt.format != format {
+				t.Fatalf("expected format: '%s', got format: '%s'", tt.format, format)
+			}
+		})
+	}
+}