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-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpRequest.java

@@ -23,6 +23,7 @@ import java.net.InetSocketAddress;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.nio.ByteBuffer;
+import java.nio.charset.Charset;
 import java.util.function.IntPredicate;
 import javax.net.ssl.SSLSession;
 
@@ -294,6 +295,11 @@ class UndertowServerHttpRequest extends AbstractServerHttpRequest {
 			return this.dataBuffer.capacity(newCapacity);
 		}
 
+		@Override
+		public DataBuffer ensureCapacity(int capacity) {
+			return this.dataBuffer.ensureCapacity(capacity);
+		}
+
 		@Override
 		public byte getByte(int index) {
 			return this.dataBuffer.getByte(index);
@@ -343,6 +349,11 @@ class UndertowServerHttpRequest extends AbstractServerHttpRequest {
 			return this.dataBuffer.write(byteBuffers);
 		}
 
+		@Override
+		public DataBuffer write(CharSequence charSequence, Charset charset) {
+			return this.dataBuffer.write(charSequence, charset);
+		}
+
 		@Override
 		public DataBuffer slice(int index, int length) {
 			return this.dataBuffer.slice(index, length);