fix: ensure content type is set for 406 not acceptable

Author: danielgtaylor

huma.go

@@ -490,6 +490,11 @@ func writeResponse(api API, ctx Context, status int, ct string, body any) error
 		ct, err = api.Negotiate(ctx.Header("Accept"))
 		if err != nil {
 			notAccept := NewErrorWithContext(ctx, http.StatusNotAcceptable, "unable to marshal response", err)
+			ct := "application/json"
+			if ctf, ok := notAccept.(ContentTypeFilter); ok {
+				ct = ctf.ContentType(ct)
+			}
+			ctx.SetHeader("Content-Type", ct)
 			if e := transformAndWrite(api, ctx, http.StatusNotAcceptable, "application/json", notAccept); e != nil {
 				return e
 			}

huma_test.go

@@ -2067,7 +2067,7 @@ Content-Type: text/plain
 			URL: "/response-accept",
 			Assert: func(t *testing.T, resp *httptest.ResponseRecorder) {
 				assert.Equal(t, http.StatusNotAcceptable, resp.Code)
-				assert.Equal(t, "application/json", resp.Header().Get("Content-Type"))
+				assert.Equal(t, "application/problem+json", resp.Header().Get("Content-Type"))
 				assert.JSONEq(t, `{"title":"Not Acceptable","status":406,"detail":"unable to marshal response","errors":[{"message":"unknown accept content type"}]}`, resp.Body.String())
 			},
 		},