bug 1507428: webdriver: fix incorrect Content-Type header; r=whimboo

The Content-Type header contains the correct MIME type, but the
charset option is "utf8" and invalid: it should be "utf-8".

Differential Revision: https://phabricator.services.mozilla.com/D12116

--HG--
extra : moz-landing-system : lando

testing/webdriver/src/server.rs

@@ -226,7 +226,7 @@ impl<U: WebDriverExtensionRoute + 'static> Service for HttpHandler<U> {
 
             let response = Response::builder()
                 .status(status)
-                .header(http::header::CONTENT_TYPE, "application/json; charset=utf8")
+                .header(http::header::CONTENT_TYPE, "application/json; charset=utf-8")
                 .header(http::header::CACHE_CONTROL, "no-cache")
                 .body(resp_body.into())
                 .unwrap();