set header if nil
set header if nil

Will only set header content-type if it’s nil.

This enables the user to override the default.

whichdigital/active-rest-client,chibicode/active-rest-client,nathanhoel/active-rest-client,andyjeffries/active-rest-client,chenhang/active-rest-client,MatthieuKopec/active-rest-client,djpate/active-rest-client,andyjeffries/flexirest,MoKuH/active-rest-client