Remove only leading space for SSE data

Prior to this commit, all white space was trimmed from Server Sent Event
data. After this commit, only a leading space is removed (if present).

Closes gh-27473

Author: poutsma

spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageReader.java

@@ -147,7 +147,13 @@ private Object buildEvent(List<String> lines, ResolvableType valueType, boolean
 		for (String line : lines) {
 			if (line.startsWith("data:")) {
 				data = (data != null ? data : new StringBuilder());
-				data.append(line.substring(5).trim()).append('\n');
+				if (line.charAt(5) != ' ') {
+					data.append(line, 5, line.length());
+				}
+				else {
+					data.append(line, 6, line.length());
+				}
+				data.append('\n');
 			}
 			if (shouldWrap) {
 				if (line.startsWith("id:")) {

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

@@ -138,6 +138,20 @@ public void readString() {
 				.verify();
 	}
 
+	@Test
+	public void trimWhitespace() {
+		MockServerHttpRequest request = MockServerHttpRequest.post("/")
+				.body(Mono.just(stringBuffer("data: \tfoo \ndata:bar\t\n\n")));
+
+		Flux<String> data = reader.read(ResolvableType.forClass(String.class),
+				request, Collections.emptyMap()).cast(String.class);
+
+		StepVerifier.create(data)
+				.expectNext("\tfoo \nbar\t")
+				.expectComplete()
+				.verify();
+	}
+
 	@Test
 	public void readPojo() {
 		MockServerHttpRequest request = MockServerHttpRequest.post("/")