Add test for calculateCapacity

Issue: SPR-17558
Closes gh-2054

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

@@ -99,7 +99,7 @@ public final class CharSequenceEncoder extends AbstractEncoder<CharSequence> {
 		});
 	}
 
-	private int calculateCapacity(CharSequence sequence, Charset charset) {
+	int calculateCapacity(CharSequence sequence, Charset charset) {
 		float maxBytesPerChar = this.charsetToMaxBytesPerChar
 				.computeIfAbsent(charset, cs -> cs.newEncoder().maxBytesPerChar());
 		float maxBytesForSequence = sequence.length() * maxBytesPerChar;

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

@@ -16,11 +16,19 @@
 
 package org.springframework.core.codec;
 
+import java.nio.charset.Charset;
+import java.util.stream.Stream;
+
+import org.junit.Test;
 import reactor.core.publisher.Flux;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.util.MimeTypeUtils;
 
+import static java.nio.charset.StandardCharsets.ISO_8859_1;
+import static java.nio.charset.StandardCharsets.US_ASCII;
+import static java.nio.charset.StandardCharsets.UTF_16;
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.junit.Assert.*;
 
 /**
@@ -65,4 +73,17 @@ public class CharSequenceEncoderTests
 				.verifyComplete());
 	}
 
+	@Test
+	public void calculateCapacity() {
+		String sequence = "Hello World!";
+		Stream.of(UTF_8, UTF_16, ISO_8859_1, US_ASCII, Charset.forName("BIG5"))
+				.forEach(charset -> {
+					int capacity = this.encoder.calculateCapacity(sequence, charset);
+					int length = sequence.length();
+					assertTrue(String.format("%s has capacity %d; length %d", charset, capacity, length),
+							capacity >= length);
+				});
+
+	}
+
 }