Use DataBuffer.write in CharSequenceEncoder

Since SPR-17558, `DataBuffer` now offers a new method to write Strings
to them. This commit makes `CharSequenceEncoder` use that.

Issue: SPR-17558

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

@@ -63,8 +63,6 @@ public class CharSequenceEncoderTests
 				.consumeNextWith(expectString(this.foo))
 				.consumeNextWith(expectString(this.bar))
 				.verifyComplete());
-
-
 	}
 
 }