Only coerce string once.

Author: headius

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

@@ -65,7 +65,7 @@ public static Encoding toEncoding(ThreadContext context, IRubyObject enc) {
         if (!encStr.getEncoding().isAsciiCompatible()) {
             throw context.runtime.newArgumentError("invalid name encoding (non ASCII)");
         }
-        Encoding idx = context.runtime.getEncodingService().getEncodingFromObject(enc);
+        Encoding idx = context.runtime.getEncodingService().getEncodingFromObject(encStr);
         // check for missing encoding is in getEncodingFromObject
         return idx;
     }