Fix catString to always use UTF-8 and CR_VALID

* Java strings are always unicode, so we should encode as UTF-8
  for concatenation.
* The resulting encoded string should always be valid.

core/src/main/java/org/jruby/RubyString.java

@@ -1508,8 +1508,8 @@ public final int catWithCodeRange(ByteList other, int codeRange) {
     }
 
     public final RubyString catString(String str) {
-        ByteList other = encodeBytelist(str, getEncoding());
-        catWithCodeRange(other, CR_UNKNOWN);
+        ByteList other = encodeBytelist(str, UTF8);
+        catWithCodeRange(other, CR_VALID);
         return this;
     }