Correctly handle encoded angle brackets.

`HtmlToPlainText#convert_to_text` clobbers all text between the html entities "&amp;lt;" and "&amp;gt;". 

This is because we do entity replacement, converting the entities into angle brackets, and then strip out remaining tags.

This commit fixes the issue by doing entity replacement **after** we strip out tags.

Before this commit:

```ruby
Premailer.new("&lt;test&gt;", :with_html_string => true).to_plain_text
# Erroneously returns ""
```

After this commit:

```ruby
Premailer.new("&lt;test&gt;", :with_html_string => true).to_plain_text
# Correctly returns "<test>"
```