Fix non-native IO.popen env + kwargs handling

When both an env hash and a keyword arguments hash were provided
here, the env hash logic would decrement argc resulting in the
keyword arguments hash detection accessing the wrong argument. The
fix here aligns this logic with the newer PopenExecutor and ensures
the keyword arguments hash is handled first.

The argument processing logic for these two popen implementations
should be unified, but this simple fix gets popen working better
right now.

Fixes jruby#9295

Author: headius

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

@@ -4518,13 +4518,15 @@ public static IRubyObject popen(ThreadContext context, IRubyObject recv, IRubyOb
 
         int firstArg = 0;
 
-        if (argc > 0 && !TypeConverter.checkHashType(runtime, args[0]).isNil()) {
-            firstArg++;
+        // process trailing keyword arguments hash
+        if (argc > 0 && !(tmp = TypeConverter.checkHashType(runtime, args[argc - 1])).isNil()) {
+            options = (RubyHash)tmp;
             argc--;
         }
 
-        if (argc > 0 && !(tmp = TypeConverter.checkHashType(runtime, args[argc - 1])).isNil()) {
-            options = (RubyHash)tmp;
+        // process leading env hash
+        if (argc > 0 && !TypeConverter.checkHashType(runtime, args[0]).isNil()) {
+            firstArg++;
             argc--;
         }