Handle encoding checks as in strTranscode

Logic in strTranscode evolved over the years to allow same-encoding
requests to be no-ops. Those changes were never applied to
rbByteEncode, resulting in same-encoding requests triggering
errors when the transcoding subsystem saw nothing would be done.
This complicated efforts to solve jruby#8682 by passing an
encoding to the IOOutputStream constructor (ruby/json#759 and
ruby/json#760).

This patch allows using IOOutputStream and the byte[] IO API it
calls with an externally-encoded IO by passing in an expected
encoding for incoming bytes. All bytes will be treated as being
encoded properly, and if the source and destination encoding is the
same, rbByteEncode will return null to indicate no-op.

Note that this misses some functionality of strTranscode in that it
does not scrub the string for same-encoding requests.

Partially addresses ruby/json#760.

Fixes jruby#8686.

Author: headius

core/src/main/java/org/jruby/util/io/EncodingUtils.java

@@ -934,18 +934,31 @@ public static IRubyObject rbStrEncode(ThreadContext context, IRubyObject str, IR
         return strTranscode1(context, to, (RubyString) str, ecflags, ecopt, EncodingUtils::encodedDup);
     }
 
-    // rb_str_encode
+    /**
+     * A version of rbStrEncode that works directly with bytes.
+     *
+     * MRI: rb_str_encode but consuming only a byte array range and producing a ByteList.
+     *
+     * @param context
+     * @param bytes
+     * @param start
+     * @param length
+     * @param encoding
+     * @param cr
+     * @param to
+     * @param ecflags
+     * @param ecopt
+     * @return
+     */
     public static ByteList rbByteEncode(ThreadContext context, byte[] bytes, int start, int length, Encoding encoding, int cr, Encoding to, int ecflags, IRubyObject ecopt) {
         byte[] sname, dname;
 
         sname = encoding.getName();
         dname = to.getName();
 
         if (noDecorators(ecflags)) {
-            if (encoding.isAsciiCompatible() && to.isAsciiCompatible()) {
-                if (cr == StringSupport.CR_7BIT) {
-                    return null;
-                }
+            if (is7BitCompat(cr, encoding, to)) {
+                return null;
             } else if (encodingEqual(sname, dname)) {
                 return null;
             }
@@ -1106,6 +1119,12 @@ private static boolean is7BitCompat(RubyString str, Encoding denc, Encoding senc
                 && str.scanForCodeRange() == StringSupport.CR_7BIT;
     }
 
+    private static boolean is7BitCompat(int cr, Encoding denc, Encoding senc) {
+        return senc != null && denc != null
+                && senc.isAsciiCompatible() && denc.isAsciiCompatible()
+                && cr == StringSupport.CR_7BIT;
+    }
+
     private static RubyString strTranscodeScrub(ThreadContext context, IRubyObject forceEncoding, RubyString str, int ecflags, IRubyObject ecopts, TranscodeResult result, boolean explicitlyInvalidReplace, Encoding denc, Encoding senc) {
         RubyString dest = str;
         if ((ecflags & EConvFlags.INVALID_MASK) != 0 && explicitlyInvalidReplace) {