Fix regex that causes Premailer to identify HTML as URI

I came across a bug where Premailer identifies HTML content as a URI if the content includes a line that looks like a URI (i.e. a newline character followed by http://foobar.baz, https://foobar.baz, or ftp://foobar.baz). My HTML looked something like this:

```html
<html>
  <link rel="stylesheet" type="text/css" href="styles.css">
  <body>
    <p>
      Premailer identified this HTML as a URI because of the following line:
http://foobar.com
    </p>
  </body>
</html>
```

This raised an exception when Premailer tried to parse the HTML content as a URI.

The problem had to do with using the ^ character in some of the regular expressions to match the beginning of a line. Using this character would match something that looked like a URI as long as it's either at the beginning of the content (which is desired) or preceded by a new line character (which is not desired, because a URI should just be one line and anything else is probably HTML content). 

My fix simply changes the ^ regular expression character to the \A (beginning of string) character in some of the match tests of lib/premailer.rb. I also added a passing test spec that fails with the original code.