Encode strings to utf-8 that are passed to compile

lib/execjs/external_runtime.rb

@@ -4,8 +4,10 @@ module ExecJS
   class ExternalRuntime
     class Context
       def initialize(runtime, source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @runtime = runtime
-        @source  = source.respond_to?(:encode) ? source.encode("UTF-8") : source
+        @source  = source
       end
 
       def eval(source, options = {})

lib/execjs/mustang_runtime.rb

@@ -2,6 +2,8 @@ module ExecJS
   class MustangRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @v8_context = ::Mustang::Context.new
         @v8_context.eval(source)
       end

lib/execjs/ruby_racer_runtime.rb

@@ -2,6 +2,8 @@ module ExecJS
   class RubyRacerRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @v8_context = ::V8::Context.new
         @v8_context.eval(source)
       end

lib/execjs/ruby_rhino_runtime.rb

@@ -2,6 +2,8 @@ module ExecJS
   class RubyRhinoRuntime
     class Context
       def initialize(source = "")
+        source = source.encode('UTF-8') if source.respond_to?(:encode)
+
         @rhino_context = ::Rhino::Context.new
         @rhino_context.eval(source)
       end

test/test_runtime.rb

@@ -69,7 +69,7 @@ def test_encoding_compile
 
       assert_raise Encoding::UndefinedConversionError do
         binary = "\xde\xad\xbe\xef".force_encoding("BINARY")
-        @runtime.eval(binary)
+        context.eval(binary)
       end
     end
   end