use accessor for ascii charset

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/str/StringBuiltins.java

@@ -57,6 +57,7 @@
 import java.nio.charset.CharsetEncoder;
 import java.nio.charset.CodingErrorAction;
 import java.nio.charset.IllegalCharsetNameException;
+import java.nio.charset.StandardCharsets;
 import java.nio.charset.UnsupportedCharsetException;
 import java.util.Arrays;
 import java.util.List;
@@ -1611,7 +1612,7 @@ Object doGeneric(VirtualFrame frame, String left, Object right,
     @GenerateNodeFactory
     @TypeSystemReference(PythonArithmeticTypes.class)
     abstract static class IsAsciiNode extends PythonUnaryBuiltinNode {
-        private static final CharsetEncoder asciiEncoder = Charset.forName("US-ASCII").newEncoder();
+        private static final CharsetEncoder asciiEncoder = StandardCharsets.US_ASCII.newEncoder();
 
         @Specialization
         @TruffleBoundary