cache: Properly produce \n only on windows on jdk9+

Setting `line.separator` is considered bad so now it is only read during JVM startup. Preferred behavior is to override println on the PrintWriter.
See http://mail.openjdk.java.net/pipermail/core-libs-dev/2015-January/030567.html

cache/src/main/java/net/runelite/cache/util/IDClass.java

@@ -40,11 +40,10 @@ public class IDClass extends PrintWriter
 
 	public static IDClass create(File directory, String name) throws IOException
 	{
-		System.setProperty("line.separator", "\n");
 		IDClass c = new IDClass(new File(directory, name + ".java"));
 		c.println("/* This file is automatically generated. Do not edit. */");
 		c.println("package net.runelite.api;");
-		c.println("");
+		c.println();
 		c.print("public final class ");
 		c.println(name);
 		c.println("{");
@@ -62,6 +61,13 @@ public class IDClass extends PrintWriter
 		println("	public static final int " + javaName + " = " + id + ";");
 	}
 
+	@Override
+	public void println()
+	{
+		// Java 9+ only reads line.separator on startup, so we have to override it here
+		write('\n');
+	}
+
 	@Override
 	public void close()
 	{