query: honor Encoder on interface fields

query/encode.go

@@ -186,6 +186,11 @@ func reflectValue(values url.Values, val reflect.Value, scope string) error {
 			continue
 		}
 
+		// unwrap interface values so the concrete type's Encoder is used
+		if sv.Kind() == reflect.Interface && !sv.IsNil() {
+			sv = sv.Elem()
+		}
+
 		if sv.Type().Implements(encoderType) {
 			// if sv is a nil pointer and the custom encoder is defined on a non-pointer
 			// method receiver, set sv to the zero value of the underlying type

query/encode_test.go

@@ -477,6 +477,14 @@ func TestValues_CustomEncodingSlice(t *testing.T) {
 			}{(*customEncodedStrings)(&[]string{"a", "b"})},
 			url.Values{"v.0": {"a"}, "v.1": {"b"}},
 		},
+
+		// custom encoded type held in an interface field
+		{
+			struct {
+				V interface{} `url:"v"`
+			}{customEncodedStrings{"a", "b"}},
+			url.Values{"v.0": {"a"}, "v.1": {"b"}},
+		},
 	}
 
 	for _, tt := range tests {