Write CharSequence instances to DataBuffers

Prior to this commit, one could write a `CharSequence` to an existing
`DataBuffer` instance by turning it into a byte array or `ByteBuffer`
first. This had the following disadvantages:

1. Memory allocation was not efficient (not leveraging pooled memory
when available)
2. Dealing with `CharsetEncoder` is not always easy
3. `DataBuffer` implementations, like `NettyDataBuffer` can use
optimized implementations in some cases

This commit adds a new `DataBuffer#write(CharSequence, Charset)` method
for those cases and also an `ensureCapacity` method useful for checking
that the current buffer has enough capacity to write to it..

Issue: SPR-17558

spring-webflux/src/test/java/org/springframework/web/reactive/function/BodyInsertersTests.java

@@ -30,6 +30,7 @@ import java.util.Map;
 import java.util.Optional;
 
 import com.fasterxml.jackson.annotation.JsonView;
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
 import reactor.core.publisher.Flux;
@@ -44,6 +45,7 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.core.io.buffer.DefaultDataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpRange;
 import org.springframework.http.ReactiveHttpOutputMessage;
@@ -121,10 +123,11 @@ public class BodyInsertersTests {
 		MockServerHttpResponse response = new MockServerHttpResponse();
 		Mono<Void> result = inserter.insert(response, this.context);
 		StepVerifier.create(result).expectComplete().verify();
-
-		DataBuffer buffer = new DefaultDataBufferFactory().wrap(body.getBytes(UTF_8));
 		StepVerifier.create(response.getBody())
-				.expectNext(buffer)
+				.consumeNextWith(buf -> {
+					String actual = DataBufferTestUtils.dumpString(buf, UTF_8);
+					Assert.assertEquals("foo", actual);
+				})
 				.expectComplete()
 				.verify();
 	}
@@ -166,11 +169,11 @@ public class BodyInsertersTests {
 		MockServerHttpResponse response = new MockServerHttpResponse();
 		Mono<Void> result = inserter.insert(response, this.context);
 		StepVerifier.create(result).expectComplete().verify();
-
-		ByteBuffer byteBuffer = ByteBuffer.wrap("foo".getBytes(UTF_8));
-		DataBuffer buffer = new DefaultDataBufferFactory().wrap(byteBuffer);
 		StepVerifier.create(response.getBody())
-				.expectNext(buffer)
+				.consumeNextWith(buf -> {
+					String actual = DataBufferTestUtils.dumpString(buf, UTF_8);
+					Assert.assertEquals("foo", actual);
+				})
 				.expectComplete()
 				.verify();
 	}