Fix the regexp cache key rope to have the correct encoding.

Author: nirvdrum

src/main/java/org/truffleruby/core/regexp/TruffleRegexpNodes.java

@@ -962,7 +962,7 @@ public static Regex compile(RubyLanguage language, RubyDeferredWarnings rubyDefe
 
         if (language.options.REGEXP_INSTRUMENT_CREATION) {
             final RegexpCacheKey key = new RegexpCacheKey(
-                    bytes.getRope(),
+                    RopeOperations.withEncoding(bytes.getRope(), enc.jcoding),
                     enc,
                     options,
                     Hashing.NO_SEED);