Use space rate to distinguish minified files.

Minified JS files usually contain less than 2% spaces, while minified
CSS files may contain about 4% spaces. However, an unminified CSS file
may also have as low as 6% spaces, especially when it includes some
resources inline. Consequently, the division might not be appreciate
for CSS files. Even though, it will only mis-recognize a normal file
as minified for a few special cases.
Add samples and tests for minified CSS detection.