GH-499,498 Add support for SupplierExporter to control output content-type

- Add 'contentType' property to ExporterProperties to assist SupplierExporter with delegating it to function catalog
- Add additional logging and testing
- Change JsonMapper to abstract class providing special handling of conversion of Json Sting to byte[]

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/test/FunctionalExporterTests.java

@@ -52,6 +52,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 		"spring.cloud.function.web.export.sink.url=http://localhost:${my.port}",
 		"spring.cloud.function.web.export.source.url=http://localhost:${my.port}",
 		"spring.cloud.function.web.export.sink.name=origin|uppercase",
+		"spring.cloud.function.web.export.sink.contentType=text/plain",
 		"spring.cloud.function.web.export.debug=true" })
 public class FunctionalExporterTests {
 

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/source/FunctionAutoConfigurationIntegrationTests.java

@@ -85,8 +85,8 @@ public class FunctionAutoConfigurationIntegrationTests {
 		}
 		// It completed
 		assertThat(this.forwarder.isOk()).isTrue();
-		assertThat(this.app.inputs).contains("HELLO");
-		assertThat(this.app.inputs).contains("WORLD");
+		assertThat(this.app.inputs).contains("\"HELLO\"");
+		assertThat(this.app.inputs).contains("\"WORLD\"");
 	}
 
 	@EnableAutoConfiguration