Fix repeated calls to DataBuffer.write(CharSequence, Charset)

Prior to this commit, repeated calls to `DataBuffer.write(CharSequence,
Charset)` would not write the given chars to the expected position in
the original buffer.

This commit fixes that by writing to the `outBuffer.position()`, offset
by the current `DataBuffer.writePosition()`.

Fixes gh-22484

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -225,6 +225,27 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		release(buffer);
 	}
 
+	@Test
+	public void writeMultipleUtf8String() {
+
+		DataBuffer buffer = createDataBuffer(1);
+		buffer.write("abc", StandardCharsets.UTF_8);
+		assertEquals(3, buffer.readableByteCount());
+
+		buffer.write("def", StandardCharsets.UTF_8);
+		assertEquals(6, buffer.readableByteCount());
+
+		buffer.write("ghi", StandardCharsets.UTF_8);
+		assertEquals(9, buffer.readableByteCount());
+
+		byte[] result = new byte[9];
+		buffer.read(result);
+
+		assertArrayEquals("abcdefghi".getBytes(), result);
+
+		release(buffer);
+	}
+
 	@Test
 	public void inputStream() throws IOException {
 		DataBuffer buffer = createDataBuffer(4);