Allow data keys to be symbols

Small change that allows symbols to be used as keys in the data passed to ```#render```.

```ruby
t = Template.new
t.parse('{{foo}}')
t.render({foo: 'FOO!'})
```

produces the same result as

```ruby
t = Template.new
t.parse('{{foo}}')
t.render({'foo' => 'FOO!'})
```