Use explicit request for json return data during OAuth token request.

Some OAuth providers like GitHub default to return web encoded strings
instead of JSON. This enables the use of cratery with GitHub Enterprise
as OAuth provider.

Signed-off-by: Diego Dompe <[email protected]>

src/services/database/users.rs

@@ -48,6 +48,7 @@ impl Database {
                 ("client_id", &configuration.oauth_client_id),
                 ("client_secret", &configuration.oauth_client_secret),
             ])
+            .header(reqwest::header::ACCEPT, "application/json")
             .send()
             .await?;
         if !response.status().is_success() {