validate headers in WebhookForm
validate headers in WebhookForm

simplify dict transformation

harrissoerja/rapidpro,Thapelo-Tsotetsi/rapidpro,ewheeler/rapidpro,tsotetsi/textily-web,praekelt/rapidpro,ewheeler/rapidpro,harrissoerja/rapidpro,praekelt/rapidpro,reyrodrigues/EU-SMS,Thapelo-Tsotetsi/rapidpro,reyrodrigues/EU-SMS,praekelt/rapidpro,pulilab/rapidpro,ewheeler/rapidpro,pulilab/rapidpro,reyrodrigues/EU-SMS,tsotetsi/textily-web,pulilab/rapidpro,pulilab/rapidpro,tsotetsi/textily-web,Thapelo-Tsotetsi/rapidpro,tsotetsi/textily-web,tsotetsi/textily-web,harrissoerja/rapidpro,pulilab/rapidpro,ewheeler/rapidpro,praekelt/rapidpro