Align our strConvEncOpts with MRI's.

When there's an error from encoding, MRI's version of this
function just returns the incoming bytes as its result. Ours used
a path that always raises an error when transcoding fails. The new
version uses a path more in line with MRI that requires an out
buffer and returns a result, so we can handle it appropriately.

Fixes #2419.

core/src/main/java/org/jruby/util/encoding/CharsetTranscoder.java

@@ -235,7 +235,21 @@ public RubyCoderResult transcode(ThreadContext context, ByteList value, ByteList
 
         return transcode(context, value, dest, fromEncoding, false, true);
     }
-
+
+    public RubyCoderResult econvConvert(ThreadContext context, ByteList inBuffer, ByteList outBuffer) {
+        Encoding fromEncoding = this.inEncoding != null ? this.inEncoding : inBuffer.getEncoding();
+
+        primitiveConvert(context, inBuffer.shallowDup(), outBuffer, 0, -1, fromEncoding, false, actions.ecflags);
+
+        if (lastResult != null) {
+            createLastError();
+        } else {
+            outBuffer.append(finish(inBuffer.getEncoding()));
+        }
+
+        return lastResult;
+    }
+
     public ByteList transcode(ThreadContext context, ByteList value) {
         ByteList dest = new ByteList();
 

core/src/main/java/org/jruby/util/encoding/Transcoder.java

@@ -149,12 +149,17 @@ public static ByteList strConvEncOpts(ThreadContext context, ByteList value, Enc
 
         Transcoder ec = EncodingUtils.econvOpenOpts(context, fromEncoding.getName(), toEncoding.getName(), ecflags, ecopts);
         if (ec == null) return value;
-
-        ByteList ret = ec.convert(context, value, false);
-
-        ret.setEncoding(toEncoding);
-
-        return ret;
+
+        ByteList newStr = new ByteList();
+        RubyCoderResult ret = ec.econvConvert(context, value, newStr);
+
+        if (ret == null || ret.stringResult.equals("finished")) {
+            newStr.setEncoding(toEncoding);
+            return newStr;
+        } else {
+            // error result, failover to original
+            return value;
+        }
     }
 
     // rb_str_conv_enc
@@ -187,6 +192,8 @@ public static ByteList transcode(ThreadContext context, ByteList value, Encoding
 
     // rb_econv_convert
     public abstract RubyCoderResult transcode(ThreadContext context, ByteList value, ByteList dest);
+
+    public abstract RubyCoderResult econvConvert(ThreadContext context, ByteList value, ByteList dest);
 
     public abstract ByteList transcode(ThreadContext context, ByteList value);