Improve default content type handling

We now also check the default content type if the content type is
application/octet-stream as we do today.

Uncommented failing test that now passes.

spring-web-reactive/src/main/java/org/springframework/http/converter/reactive/CodecHttpMessageConverter.java

@@ -132,14 +132,22 @@ public class CodecHttpMessageConverter<T> implements HttpMessageConverter<T> {
 		if (this.encoder == null) {
 			return Mono.error(new IllegalStateException("No decoder set"));
 		}
+
 		HttpHeaders headers = outputMessage.getHeaders();
 		if (headers.getContentType() == null) {
 			MediaType contentTypeToUse = contentType;
 			if (contentType == null || contentType.isWildcardType() || contentType.isWildcardSubtype()) {
 				contentTypeToUse = getDefaultContentType(type);
 			}
-			headers.setContentType(contentTypeToUse);
+			else if (MediaType.APPLICATION_OCTET_STREAM.equals(contentType)) {
+				MediaType mediaType = getDefaultContentType(type);
+				contentTypeToUse = (mediaType != null ? mediaType : contentTypeToUse);
+			}
+			if (contentTypeToUse != null) {
+				headers.setContentType(contentTypeToUse);
+			}
 		}
+
 		DataBufferFactory bufferFactory = outputMessage.bufferFactory();
 		Flux<DataBuffer> body = this.encoder.encode(inputStream, bufferFactory, type, contentType);
 		return outputMessage.writeWith(body);

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/MessageConverterResultHandlerTests.java

@@ -48,6 +48,7 @@ import org.springframework.core.convert.support.GenericConversionService;
 import org.springframework.core.convert.support.ReactiveStreamsToCompletableFutureConverter;
 import org.springframework.core.convert.support.ReactiveStreamsToRxJava1Converter;
 import org.springframework.core.io.ByteArrayResource;
+import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.HttpMethod;
@@ -90,16 +91,12 @@ public class MessageConverterResultHandlerTests {
 
 
 	@Test // SPR-12894
-	@Ignore // GH # 121
 	public void useDefaultContentType() throws Exception {
-		Object body = new ByteArrayResource("body".getBytes("UTF-8"));
+		Resource body = new ClassPathResource("logo.png", getClass());
 		ResolvableType bodyType = ResolvableType.forType(Resource.class);
 		this.resultHandler.writeBody(this.exchange, body, bodyType).block(Duration.ofSeconds(5));
 
-		assertEquals("image/jpeg", this.response.getHeaders().getFirst("Content-Type"));
-		TestSubscriber.subscribe(this.response.getBody())
-				.assertValuesWith(buf -> assertEquals("body",
-						DataBufferTestUtils.dumpString(buf, Charset.forName("UTF-8"))));
+		assertEquals("image/x-png", this.response.getHeaders().getFirst("Content-Type"));
 	}
 
 	@Test