GH-1235 Fix support for text/* contentType

Resolves #1235

Author: olegz

spring-cloud-function-adapters/spring-cloud-function-adapter-gcp/src/main/java/org/springframework/cloud/function/adapter/gcp/FunctionInvoker.java

@@ -117,7 +117,7 @@ public void service(HttpRequest httpRequest, HttpResponse httpResponse) throws E
 		Message<BufferedReader> message = this.functionWrapped.getInputType() == Void.class || this.functionWrapped.getInputType() == null ? null
 			: MessageBuilder.withPayload(httpRequest.getReader()).copyHeaders(httpRequest.getHeaders()).build();
 
-		Message<byte[]> result = function.apply(message);
+		Message<?> result = function.apply(message);
 
 		if (result != null) {
 			MessageHeaders headers = result.getHeaders();
@@ -130,7 +130,8 @@ else if (result.getHeaders().containsKey("Content-Type")) {
 			else {
 				httpRequest.getContentType().ifPresent(contentType -> httpResponse.setContentType(contentType));
 			}
-			httpResponse.getWriter().write(new String(result.getPayload(), StandardCharsets.UTF_8));
+			String content = result.getPayload() instanceof String strPayload ? strPayload : new String((byte[]) result.getPayload(), StandardCharsets.UTF_8);
+			httpResponse.getWriter().write(content);
 			for (Entry<String, Object> header : headers.entrySet()) {
 				Object values = header.getValue();
 				if (values instanceof Collection<?>) {

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/ContextFunctionCatalogAutoConfiguration.java

@@ -55,7 +55,6 @@
 import org.springframework.cloud.function.json.GsonMapper;
 import org.springframework.cloud.function.json.JacksonMapper;
 import org.springframework.cloud.function.json.JsonMapper;
-import org.springframework.cloud.function.utils.PrimitiveTypesFromStringMessageConverter;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Bean;
@@ -71,13 +70,17 @@
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.ByteArrayMessageConverter;
 import org.springframework.messaging.converter.CompositeMessageConverter;
+import org.springframework.messaging.converter.ContentTypeResolver;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.converter.StringMessageConverter;
 import org.springframework.stereotype.Component;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.InvalidMimeTypeException;
+import org.springframework.util.MimeType;
 import org.springframework.util.StringUtils;
 
 /**
@@ -133,8 +136,23 @@ public FunctionRegistry functionCatalog(List<MessageConverter> messageConverters
 
 		mcList.add(new JsonMessageConverter(jsonMapper));
 		mcList.add(new ByteArrayMessageConverter());
-		mcList.add(new StringMessageConverter());
-		mcList.add(new PrimitiveTypesFromStringMessageConverter(conversionService));
+		StringMessageConverter stringConverter = new StringMessageConverter();
+		stringConverter.setSerializedPayloadClass(String.class);
+		stringConverter.setContentTypeResolver(new ContentTypeResolver() {
+			@Override
+			public MimeType resolve(MessageHeaders headers) throws InvalidMimeTypeException {
+				if (headers.containsKey(MessageHeaders.CONTENT_TYPE)) {
+					if (headers.get(MessageHeaders.CONTENT_TYPE).toString().startsWith("text")) {
+						return MimeType.valueOf("text/plain");
+					}
+					else {
+						return MimeType.valueOf(headers.get(MessageHeaders.CONTENT_TYPE).toString());
+					}
+				}
+				return null;
+			}
+		});
+		mcList.add(stringConverter);
 
 		messageConverter = new SmartCompositeMessageConverter(mcList, () -> {
 			return context.getBeansOfType(MessageConverterHelper.class).values();