Fix disable_metadata_checks behavior and fix StaticItem support of bytes content

[benoit74]

Rationale

• libzim StringProvider supports both bytes and str types as content but StaticItem accepted only str in its type hint
  • this was detected while migrating add_item_for to use new StaticItem parameters
• last PR #135 was incorrect 🤦🏻
• using main code on kolibri showed an issue when the content passed to add_item_for/StaticItem is an empty string "" ; content is not set and StaticItem fails to know which content provider has to be used

Changes

• accept both bytes and str types as content in StaticItem
• accept both bytes and str types as content in scraperlib StringProvider
• check at runtime that content type is correct before passing it to the StringProvider
• reverse/fix logic of disable_metadata_checks
• always set item attributes when passed, even if 'False'/empty/whatever
• add sufficient tests, including the addition of a non-UTF8 item into the ZIM (as bytes) and its decoding

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a25ec60) 100.00% compared to head (d180e35) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #136 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 32 32 Lines 1373 1374 +1 Branches 239 240 +1 ========================================= + Hits 1373 1374 +1 ```

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