Reduce memory footprint of AsciiBytes.hashCode

Update `AsciiBytes.hashCode(int hash, String string)` so that it no
longer copies the backing array of the string.

Closes gh-7851

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/AsciiBytes.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 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.
@@ -224,9 +224,8 @@ final class AsciiBytes {
 	}
 
 	public static int hashCode(int hash, String string) {
-		char[] chars = string.toCharArray();
-		for (int i = 0; i < chars.length; i++) {
-			hash = 31 * hash + chars[i];
+		for (int i = 0; i < string.length(); i++) {
+			hash = 31 * hash + string.charAt(i);
 		}
 		return hash;
 	}