Fix AbstractJackson2Encoder#canEncode

This commit ensures that the Jackson encoder is selected only
with supported mime types.

Issue: SPR-15910

Author: sdeleuze

spring-web/src/main/java/org/springframework/http/codec/json/AbstractJackson2Encoder.java

@@ -79,9 +79,8 @@ public void setStreamingMediaTypes(List<MediaType> mediaTypes) {
 	@Override
 	public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType) {
 		Class<?> clazz = elementType.resolve(Object.class);
-		return (Object.class == clazz) ||
-				!String.class.isAssignableFrom(elementType.resolve(clazz)) &&
-						getObjectMapper().canSerialize(clazz) && supportsMimeType(mimeType);
+		return supportsMimeType(mimeType) && (Object.class == clazz ||
+				(!String.class.isAssignableFrom(elementType.resolve(clazz)) && getObjectMapper().canSerialize(clazz)));
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonEncoderTests.java

@@ -61,6 +61,9 @@ public void canEncode() {
 
 		// SPR-15464
 		assertTrue(this.encoder.canEncode(ResolvableType.NONE, null));
+
+		// SPR-15910
+		assertFalse(this.encoder.canEncode(ResolvableType.forClass(Object.class), APPLICATION_OCTET_STREAM));
 	}
 
 	@Test // SPR-15866