Restore max-http-header-size default value support

Fix `TomcatWebServerFactoryCustomizer` to restore Spring Boot 2.0
behavior where a negative or zero `max-http-header-size` indicates
that the server default should be used.

See gh-14986

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizer.java

@@ -86,7 +86,7 @@ public class TomcatWebServerFactoryCustomizer implements
 		propertyMapper.from(tomcatProperties::getMinSpareThreads).when(this::isPositive)
 				.to((minSpareThreads) -> customizeMinThreads(factory, minSpareThreads));
 		propertyMapper.from(this::determineMaxHttpHeaderSize).whenNonNull()
-				.asInt(DataSize::toBytes)
+				.asInt(DataSize::toBytes).when(this::isPositive)
 				.to((maxHttpHeaderSize) -> customizeMaxHttpHeaderSize(factory,
 						maxHttpHeaderSize));
 		propertyMapper.from(tomcatProperties::getMaxSwallowSize).whenNonNull()

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/TomcatWebServerFactoryCustomizerTests.java

@@ -129,6 +129,22 @@ public class TomcatWebServerFactoryCustomizerTests {
 						.isEqualTo(DataSize.ofKilobytes(1).toBytes()));
 	}
 
+	@Test
+	public void customMaxHttpHeaderSizeIgnoredIfNegative() {
+		bind("server.max-http-header-size=-1");
+		customizeAndRunServer((server) -> assertThat(((AbstractHttp11Protocol<?>) server
+				.getTomcat().getConnector().getProtocolHandler()).getMaxHttpHeaderSize())
+						.isEqualTo(DataSize.ofKilobytes(8).toBytes()));
+	}
+
+	@Test
+	public void customMaxHttpHeaderSizeIgnoredIfZero() {
+		bind("server.max-http-header-size=0");
+		customizeAndRunServer((server) -> assertThat(((AbstractHttp11Protocol<?>) server
+				.getTomcat().getConnector().getProtocolHandler()).getMaxHttpHeaderSize())
+						.isEqualTo(DataSize.ofKilobytes(8).toBytes()));
+	}
+
 	@Test
 	@Deprecated
 	public void customMaxHttpHeaderSizeWithDeprecatedProperty() {