[DomCrawler] Fixed charset detection in html5 meta charset tag

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT

It may be minor to folks with ascii-charactered language, but is critical for us Japanese.
Many Japanese websites with SJIS encoding have "Shift_JIS" as their encoding declaration.