Try to convert to UTF8 early

lib/execjs/external_runtime.rb

@@ -9,12 +9,16 @@ module ExecJS
       end
 
       def eval(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           exec("return eval(#{MultiJson.encode("(#{source})")})")
         end
       end
 
       def exec(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         compile_to_tempfile([@source, source].join("\n")) do |file|
           extract_result(@runtime.send(:exec_runtime, file.path))
         end

lib/execjs/mustang_runtime.rb

@@ -7,12 +7,16 @@ module ExecJS
       end
 
       def exec(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           eval "(function(){#{source}})()", options
         end
       end
 
       def eval(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           unbox @v8_context.eval("(#{source})")
         end

lib/execjs/ruby_racer_runtime.rb

@@ -7,12 +7,16 @@ module ExecJS
       end
 
       def exec(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           eval "(function(){#{source}})()", options
         end
       end
 
       def eval(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           unbox @v8_context.eval("(#{source})")
         end

lib/execjs/ruby_rhino_runtime.rb

@@ -7,12 +7,16 @@ module ExecJS
       end
 
       def exec(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           eval "(function(){#{source}})()", options
         end
       end
 
       def eval(source, options = {})
+        souce = source.encode('UTF-8') if source.respond_to?(:encode)
+
         if /\S/ =~ source
           unbox @rhino_context.eval("(#{source})")
         end

test/test_runtimes.rb

@@ -36,8 +36,19 @@ module TestRuntime
   if defined? Encoding
     def test_encoding
       utf8 = Encoding.find('UTF-8')
+
       assert_equal utf8, @runtime.exec("return 'hello'").encoding
       assert_equal utf8, @runtime.eval("'☃'").encoding
+
+      ascii = "'hello'".encode('US-ASCII')
+      result = @runtime.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