Allow PORTFILE to always override the file to use

Previously, the `PORTFILE` system property was not checked if the
`EmbeddedServerPortFileWriter` was created using the default constructor.

This had the effect to prevent overriding of the port file when this
listener is created without any file or via `META-INF/spring.factories`.

Closes gh-4254

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/system/EmbeddedServerPortFileWriter.java

 /*
- * Copyright 2010-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -57,7 +57,7 @@ public class EmbeddedServerPortFileWriter
 	 * 'application.port'.
 	 */
 	public EmbeddedServerPortFileWriter() {
-		this.file = new File(DEFAULT_FILE_NAME);
+		this(new File(DEFAULT_FILE_NAME));
 	}
 
 	/**

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/system/EmbeddedServerPortFileWriterTests.java

 /*
- * Copyright 2010-2014 the original author or authors.
+ * Copyright 2012-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -66,7 +66,16 @@ public class EmbeddedServerPortFileWriterTests {
 	}
 
 	@Test
-	public void overridePortFile() throws Exception {
+	public void overridePortFileWithDefault() throws Exception {
+		System.setProperty("PORTFILE", this.temporaryFolder.newFile().getAbsolutePath());
+		EmbeddedServerPortFileWriter listener = new EmbeddedServerPortFileWriter();
+		listener.onApplicationEvent(mockEvent("", 8080));
+		assertThat(FileCopyUtils.copyToString(
+				new FileReader(System.getProperty("PORTFILE"))), equalTo("8080"));
+	}
+
+	@Test
+	public void overridePortFileWithExplicitFile() throws Exception {
 		File file = this.temporaryFolder.newFile();
 		System.setProperty("PORTFILE", this.temporaryFolder.newFile().getAbsolutePath());
 		EmbeddedServerPortFileWriter listener = new EmbeddedServerPortFileWriter(file);