fix File.readable? when native is disabled

this readable? method now follows the same pattern as writable?, file?,
directory? and exists? and uses the respective FileResource directly.

fixes #2040 and #1983

Sponsored by Lookout Inc.

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

@@ -197,8 +197,7 @@ public static IRubyObject readable_p(ThreadContext context, IRubyObject recv, IR
             filename = get_path(context, filename);
         }
 
-        FileStat stat = fileResource(filename).stat();
-        return runtime.newBoolean(stat != null && stat.isReadable());
+        return runtime.newBoolean(fileResource(filename).canRead());
     }
 
     // Not exposed by filetest, but so similiar in nature that it is stored here

test/test_file.rb

@@ -484,6 +484,14 @@ def test_file_query # - file?
     assert(result == 'falsefalsetrue')
   end
 
+  def test_readable_query # - readable?
+    assert(File.readable?('test/test_file.rb'))
+    assert(File.readable?('test'))
+    assert(! File.readable?('test_not'))
+    result = jruby("-e 'print File.readable?(\"test_not\");print File.readable?(\"test\");print File.readable?(\"test/test_file.rb\")'", 'jruby.native.enabled' => 'false' )
+    assert(result == 'falsetruetrue')
+  end
+
   def test_file_exist_query
     assert(File.exist?('test'))
     assert(! File.exist?('test_not'))