resolved issue where non tag content is being removed

Please view the test that was added in `test_html_to_plain_text.rb`:

```ruby
def test_to_plain_text_with_non_tag_gt_lt_characters
  html = "<p>Here is some content with a < (less than) and > (greater than) sign</p>"
  premailer = Premailer.new(html, :with_html_string => true, :line_length => 10000)
  assert_equal "Here is some content with a < (less than) and > (greater than) sign", premailer.to_plain_text
end
```

This was causing us issues where the content `< (less than) and >` was being removed completely.