Allow the tomcat maxHttpHeaderSize to be changed in external config

Prevents large SPNEGO headers from causing server errors for example.
Added the property to ServerProperties.

Fixes gh-931

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/ServerPropertiesTests.java

@@ -34,7 +34,7 @@ import static org.mockito.Mockito.verify;
 
 /**
  * Tests for {@link ServerProperties}.
- * 
+ *
  * @author Dave Syer
  * @author Stephane Nicoll
  */
@@ -97,4 +97,13 @@ public class ServerPropertiesTests {
 		assertEquals("US-ASCII", this.properties.getTomcat().getUriEncoding());
 	}
 
+	@Test
+    public void testCustomizeTomcatHeaderSize() throws Exception {
+		Map<String, String> map = new HashMap<String, String>();
+		map.put("server.tomcat.maxHttpHeaderSize", "9999");
+		new RelaxedDataBinder(this.properties, "server").bind(new MutablePropertyValues(
+				map));
+		assertEquals(9999, this.properties.getTomcat().getMaxHttpHeaderSize());
+	}
+
 }