Call static::dumpAsString in VarDumper::dump

This way we can extend VarDumper::dumpAsString
Merge branch 'master' of git://github.com/yiisoft/yii2 into var-dumper-dump-call-static-dump-as-string
Merge pull request #121 from rinatio/var-dumper-dump-call-static-dump-as-string

Var dumper dump call static dump as string