Avoid copying native ropes when setting encoding.

Author: aardvark179

src/main/java/org/truffleruby/core/rope/NativeRope.java

@@ -178,7 +178,7 @@ public String toString() {
 
     @Override
     public Rope withEncoding(Encoding newEncoding, CodeRange newCodeRange) {
-        return RopeOperations.create(getBytes(), newEncoding, newCodeRange);
+        return new NativeRope(pointer, byteLength(), newEncoding, UNKNOWN_CHARACTER_LENGTH, CodeRange.CR_UNKNOWN);
     }
 
     public Pointer getNativePointer() {

src/main/java/org/truffleruby/core/rope/RopeNodes.java

@@ -976,7 +976,7 @@ protected Rope withEncodingSameEncoding(Rope rope, Encoding encoding) {
 
         @Specialization(guards = "rope.getEncoding() != encoding")
         protected Rope nativeRopeWithEncoding(NativeRope rope, Encoding encoding) {
-            return rescanBytesForEncoding(rope, encoding);
+            return rope.withEncoding(encoding, rope.getCodeRange());
         }
 
         @Specialization(guards = {