Rename preferred-mapper property so its clear it only applies to JSON

preferred-mapper has been renamed to preferred-json-mapper to make it
clear that it only affects the JSON mapper and that the XML mapper
that will be used (if any) is unaffected

See gh-2247

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/HttpMessageConvertersAutoConfigurationTests.java

@@ -148,7 +148,7 @@ public class HttpMessageConvertersAutoConfigurationTests {
 				JacksonAutoConfiguration.class,
 				HttpMessageConvertersAutoConfiguration.class);
 		EnvironmentTestUtils.addEnvironment(this.context,
-				"spring.http.converters.preferred-mapper:gson");
+				"spring.http.converters.preferred-json-mapper:gson");
 		this.context.refresh();
 		assertConverterBeanExists(GsonHttpMessageConverter.class,
 				"gsonHttpMessageConverter");