Ensure StringDecoder supports multiline delimiters

This commit makes sure the StringDecoder supports stripping off
multi-line delimiters, such as \r\n. Specifically, we ensure that the
delimiter is stripped from the joined buffer.

Closes gh-26511

Author: poutsma

spring-core/src/main/java/org/springframework/core/codec/StringDecoder.java

@@ -157,16 +157,20 @@ private Collection<DataBuffer> processDataBuffer(
 				int startIndex = buffer.readPosition();
 				int length = (endIndex - startIndex + 1);
 				DataBuffer slice = buffer.retainedSlice(startIndex, length);
-				if (this.stripDelimiter) {
-					slice.writePosition(slice.writePosition() - matcher.delimiter().length);
-				}
 				result = (result != null ? result : new ArrayList<>());
 				if (chunks.isEmpty()) {
+					if (this.stripDelimiter) {
+						slice.writePosition(slice.writePosition() - matcher.delimiter().length);
+					}
 					result.add(slice);
 				}
 				else {
 					chunks.add(slice);
-					result.add(buffer.factory().join(chunks));
+					DataBuffer joined = buffer.factory().join(chunks);
+					if (this.stripDelimiter) {
+						joined.writePosition(joined.writePosition() - matcher.delimiter().length);
+					}
+					result.add(joined);
 					chunks.clear();
 				}
 				buffer.readPosition(endIndex + 1);

spring-core/src/test/java/org/springframework/core/codec/StringDecoderTests.java

@@ -125,17 +125,32 @@ void decodeNewLine() {
 		);
 
 		testDecode(input, String.class, step -> step
-				.expectNext("")
+				.expectNext("").as("1st")
 				.expectNext("abc")
 				.expectNext("defghi")
-				.expectNext("")
+				.expectNext("").as("2nd")
 				.expectNext("jklmno")
 				.expectNext("pqr")
 				.expectNext("stuvwxyz")
 				.expectComplete()
 				.verify());
 	}
 
+	@Test
+	void decodeNewlinesAcrossBuffers()  {
+		Flux<DataBuffer> input = Flux.just(
+				stringBuffer("\r"),
+				stringBuffer("\n"),
+				stringBuffer("xyz")
+		);
+
+		testDecode(input, String.class, step -> step
+				.expectNext("")
+				.expectNext("xyz")
+				.expectComplete()
+				.verify());
+	}
+
 	@Test
 	void maxInMemoryLimit() {
 		Flux<DataBuffer> input = Flux.just(