@Test
    public void testStringHeaderToJson() {
        JsonNode converted = parse(converter.fromConnectHeader(TOPIC, "headerName", Schema.STRING_SCHEMA, "test-string"));
        validateEnvelope(converted);
        assertEquals(parse("{ \"type\": \"string\", \"optional\": false }"), converted.get(JsonSchema.ENVELOPE_SCHEMA_FIELD_NAME));
        assertEquals("test-string", converted.get(JsonSchema.ENVELOPE_PAYLOAD_FIELD_NAME).textValue());
    }
When schemas are disabled, fields are mapped to Connect maps.
connect/json/src/test/java/org/apache/kafka/connect/json/JsonConverterTest.java