Check arity to support existing runtime classes

lib/execjs/runtime.rb

@@ -31,17 +31,31 @@ def context_class
     end
 
     def exec(source, options = {})
-      context = context_class.new(self)
-      context.exec(source, options)
+      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.eval(source, options)
+      context = compile("", options)
+
+      if context.method(:eval).arity == 1
+        context.eval(source)
+      else
+        context.eval(source, options)
+      end
     end
 
     def compile(source, options = {})
-      context_class.new(self, 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?