`SerializationFeature.WRITE_BIGDECIMAL_AS_PLAIN` not taken into account when serializing `BigDecimal` as String

The following test should convert a BigDecimal number to "3.033" string in json. But in fact it only converts it to double.

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import org.junit.Test;

import java.math.BigDecimal;

import static org.junit.Assert.assertEquals;

public class JacksonTest {
	public class MyDto {
		private BigDecimal number;

		public BigDecimal getNumber() {
			return number;
		}

		public void setNumber(BigDecimal number) {
			this.number = number;
		}
	}

	@Test
	public void testBigDecimalSerialization() throws Exception {
		ObjectMapper mapper = new ObjectMapper().enable(SerializationFeature.WRITE_BIGDECIMAL_AS_PLAIN);
		MyDto dto = new MyDto();
		dto.number = new BigDecimal("3.033");
		String json = mapper.writeValueAsString(dto);

		assertEquals("{\"number\":\"3.033\"}", json);
	}
}

Result:

org.junit.ComparisonFailure: 
Expected :{"number":"3.033"}
Actual   :{"number":3.033}

jackson-databind-2.9.8