Added proper Content-Type header for 4xx error

src/utils/api.js

@@ -20,7 +20,9 @@ module.exports = {
 					await requestValidator(req, res, _next);
 					if (nextWatcher.wasCalled) return nextWatcher.doNext();
 				} catch (e) { // Probably bad request
-					return res.status(400).send(e.message);
+					return res.status(400)
+						.set('Content-Type', 'text/plain')
+						.send(e.message);
 				}
 			}
 			
@@ -30,7 +32,9 @@ module.exports = {
 					if (nextWatcher.wasCalled) return nextWatcher.doNext();
 				} catch (e) { // Operation error
 					const code = e.code || (e.response && e.response.code);
-					if (/^4\d\d/.test(code)) return res.status(code).send(e.message);
+					if (/^4\d\d/.test(code)) return res.status(code)
+						.set('Content-Type', 'text/plain')
+						.send(e.message);
 					
 					return next(e);
 				}