Fix behavior of content_tag when use with content that is not a string

If you pass a number in content_tag, It is considered as a codepoint.
maybe this behavior is not intended in many cases.
(ex: If you pass an array of numeric options to the select_tag)

This PR is change it to be treated as a string.
What do you think to that?

```
before:
  content_tag(:p, 97) #=> <p>a</p>

after:
  content_tag(:p, 97) #=> <p>97</p>
```