Add encoding, string path for checkEncoding

The stringio library depends on being able to pass an encoding
only as the first argument to rb_enc_check, which causes the
downstream enc_compatible_latter to reject incompatible encodings
regardless of the first string being blank.

Part of fixes to pass stringio tests in ruby/stringio#116.

Author: headius

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

@@ -53,6 +53,7 @@
 import org.jruby.runtime.encoding.EncodingService;
 import org.jruby.runtime.opto.OptoFactory;
 import org.jruby.util.ByteList;
+import org.jruby.util.CodeRangeable;
 import org.jruby.util.StringSupport;
 import org.jruby.util.io.EncodingUtils;
 
@@ -473,6 +474,13 @@ private static RawCoder getRawCoder() {
         return coder;
     }
 
+    public static Encoding checkEncoding(ThreadContext context, Encoding encoding, CodeRangeable other) {
+        Encoding enc = StringSupport.areCompatible(encoding, other);
+        if (enc == null) throw context.runtime.newEncodingCompatibilityError("incompatible character encodings: " +
+                encoding + " and " + other.getByteList().getEncoding());
+        return enc;
+    }
+
     @JRubyMethod(name = "list", meta = true)
     public static IRubyObject list(ThreadContext context, IRubyObject recv) {
         Ruby runtime = context.runtime;

core/src/main/java/org/jruby/util/StringSupport.java

@@ -1946,6 +1946,15 @@ public static Encoding areCompatible(CodeRangeable str1, CodeRangeable str2) {
         str2.scanForCodeRange();
         return encCompatibleLatter(str1, str2, enc1, enc2);
     }
+
+    public static Encoding areCompatible(Encoding enc1, CodeRangeable str2) {
+        Encoding enc2 = str2.getByteList().getEncoding();
+
+        if (enc1 == enc2) return enc1;
+
+        str2.scanForCodeRange();
+        return encCompatibleLatter(null, str2, enc1, enc2);
+    }
 
     private static Encoding encCompatibleLatter(CodeRangeable str1, CodeRangeable str2, Encoding enc1, Encoding enc2) {
         boolean isstr1, isstr2;
@@ -1972,9 +1981,9 @@ private static Encoding encCompatibleLatter(CodeRangeable str1, CodeRangeable st
 
         if (!isstr1) {
             CodeRangeable tmp = str1;
-            Encoding enc0 = enc1;
             str1 = str2;
             str2 = tmp;
+            Encoding enc0 = enc1;
             enc1 = enc2;
             enc2 = enc0;
             boolean tmp2 = isstr1;