Fix CakeHtmlReporter output for HTML

This improves the output of the Webtestrunner.
Currently, output for HTML in the diff would not show:
```
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
 '
-'
+'
```

With this patch it now is visible:
```
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
 '<link href="http://example.com/some/url" rel="alternate" hreflang="de" />
-<link href="http://localhost/some/url" rel="alternate" hreflang="de-ch" />'
+<link href="http://example.com/some/url" rel="alternate" hreflang="de-ch" />'
```