index.js
function (field, value) {
        if (is(field).a(Object)) {
          for (var key in field) {
            if (Object.prototype.hasOwnProperty.call(field, key)) {
              $this.header(key, field[key])
            }
          }

          return $this
        }

        var existingHeaderName = $this.hasHeader(field)
        $this.options.headers[existingHeaderName || field] = value

        return $this
      }
function (field, value) {
        if (is(field).a(Object)) {
          for (var key in field) {
            if (Object.prototype.hasOwnProperty.call(field, key)) {
              $this.header(key, field[key])
            }
          }

          return $this
        }

        var existingHeaderName = $this.hasHeader(field)
        $this.options.headers[existingHeaderName || field] = value

        return $this
      }
function ( field , value ) { if ( is ( field ) . a ( Object ) ) { for ( var key in field ) { if ( Object . prototype . hasOwnProperty . call ( field , key ) ) { $this . header ( key , field [ key ] ) } } return $this } var existingHeaderName = $this . hasHeader ( field ) $this . options . headers [ existingHeaderName || field ] = value return $this }
Sets header field to value

@param  {String} field Header field
@param  {String} value Header field value
@return {Object}
Sets header field to value