Convert Symbol explicitly to a JS String and add test

lib/execjs/graaljs_runtime.rb

@@ -98,10 +98,8 @@ module ExecJS
         case value
         when nil, true, false, Integer, Float
           value
-        when String
+        when String, Symbol
           Truffle::Interop.as_truffle_string value
-        when Symbol
-          value.to_s
         when Array
           value.map { |e| convert_ruby_to_js(e) }
         when Hash

test/test_execjs.rb

@@ -172,6 +172,9 @@ class TestExecJS < Test
     assert_equal "symbol", context.call("echo", :symbol)
     assert_equal ["symbol"], context.call("echo", [:symbol])
     assert_equal({"key" => "value"}, context.call("echo", {key: :value}))
+
+    context = ExecJS.compile("function myslice(str) { return str.slice(1); }")
+    assert_equal "ymbol", context.call("myslice", :symbol)
   end
 
   def test_additional_options