SSE reader support to read full content as a String

The SSE reader is ordered ahead of StringDecoder because with
response.decodeToFlux(String.class) we actually want the SSE reader
to get involved first based on the content-type.

At the same time with response.decodeToMono(String.class) there is
nothing the SSE reader can do while the StringDecoder could read the
entore content as one String, as long as the server does terminate
the stream which can happen in a testing scenario.

This commit updates ServerSentEventHttpMessageReader#readMono in
to support String.class by delegating to StringDecoder. Since
reading to a Mono is an explicit choice there is not much possibility
for interfering with decoding to Flux.

Issue: SPR-15331

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

@@ -21,6 +21,7 @@ import java.util.Collections;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -157,4 +158,18 @@ public class ServerSentEventHttpMessageReaderTests extends AbstractDataBufferAll
 				.verify();
 	}
 
+	@Test // SPR-15331
+	public void decodeFullContentAsString() {
+
+		String body = "data:foo\ndata:bar\n\ndata:baz\n\n";
+		MockServerHttpRequest request = MockServerHttpRequest.post("/").body(body);
+
+		String actual = messageReader
+				.readMono(ResolvableType.forClass(String.class), request, Collections.emptyMap())
+				.cast(String.class)
+				.block(Duration.ZERO);
+
+		assertEquals(body, actual);
+	}
+
 }