singleton, default is false, not true

[qq20004604]

https://github.com/webpack-contrib/style-loader#singleton

This option is on by default in IE9, which has strict limitations on the number of style tags allowed on a page. You can enable or disable it with the singleton option.

But in fact, singleton default is false, not true.

DEMO: https://github.com/qq20004604/webpack-study/tree/master/5%E3%80%81Loader/style_loader_demo

[alexander-akait]

@qq20004604 hm, you are right, let's fix it

[alexander-akait]

Done in #285