Encode initial source to UTF-8 when using external runtime

lib/execjs/external_runtime.rb

@@ -5,7 +5,7 @@ class ExternalRuntime
     class Context
       def initialize(runtime, source = "")
         @runtime = runtime
-        @source  = source
+        @source  = source.respond_to?(:encode) ? source.encode("UTF-8") : source
       end
 
       def eval(source, options = {})

test/test_runtime.rb

@@ -53,6 +53,25 @@ def test_encoding
         @runtime.eval(binary)
       end
     end
+
+    def test_encoding_compile
+      utf8 = Encoding.find('UTF-8')
+
+      context = @runtime.compile("foo = function(v) { return '¶' + v; }".encode("ISO8859-15"))
+
+      assert_equal utf8, context.exec("return foo('hello')").encoding
+      assert_equal utf8, context.eval("foo('☃')").encoding
+
+      ascii = "foo('hello')".encode('US-ASCII')
+      result = context.eval(ascii)
+      assert_equal "¶hello", result
+      assert_equal utf8, result.encoding
+
+      assert_raise Encoding::UndefinedConversionError do
+        binary = "\xde\xad\xbe\xef".force_encoding("BINARY")
+        @runtime.eval(binary)
+      end
+    end
   end
 
   def test_compile