add default charset of application/json to utf-8

[wanjm]

add default charset of application/json to utf-8. as. https://github.com/dart-lang/http/issues/1331

[google-cla[bot]]

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[wanjm]

@googlebot I signed it!

[Wouter12]

Any idea on when this is gonna be merged? The CI build was canceled it seems

[wanjm]

why is there error in it, how to fix it , any one help me ?

[kevmoo]

please try rebasing and pushing your fix. That should re-run CI. We've moved CI providers to GitHub