GH-1172 Ensure content-type set in a message is honored on the output

Also, ensures that the proper content type is set in GCF FunctionInvoker

Resolves #1172

spring-cloud-function-adapters/spring-cloud-function-grpc-cloudevent-ext/pom.xml

@@ -45,6 +45,16 @@
 		</extensions>
 		<plugins>
 			<plugin>
+	            <groupId>org.apache.maven.plugins</groupId>
+	            <artifactId>maven-checkstyle-plugin</artifactId>
+	            <executions>
+	                <execution>
+	                    <id>checkstyle-validation</id>
+	                    <phase>none</phase>
+	                </execution>
+	            </executions>
+	        </plugin>
+			<plugin>
 		      <groupId>org.xolstice.maven.plugins</groupId>
 		      <artifactId>protobuf-maven-plugin</artifactId>
 		      <version>0.6.1</version>