Check arity to support existing runtime classes

lib/execjs/runtime.rb

@@ -31,18 +31,32 @@ module ExecJS
     end
 
     def exec(source, options = {})
-      context = context_class.new(self)
+      context = compile("", options)
+
+      if context.method(:exec).arity == 1
+        context.exec(source)
+      else
         context.exec(source, options)
       end
+    end
 
     def eval(source, options = {})
-      context = context_class.new(self)
+      context = compile("", options)
+
+      if context.method(:eval).arity == 1
+        context.eval(source)
+      else
         context.eval(source, options)
       end
+    end
 
     def compile(source, options = {})
+      if context_class.instance_method(:initialize).arity == 2
+        context_class.new(self, source)
+      else
         context_class.new(self, source, options)
       end
+    end
 
     def deprecated?
       false