feat(api-base): set GraphQL request 'Content-Type' to 'application/json' (#391)
feat(api-base): set GraphQL request 'Content-Type' to 'application/json' (#391)

* specify header 'Content-Type' to 'application/json'

* prettier
Qminder/javascript-api,Qminder/javascript-api,Qminder/javascript-api