Use enum constants for GetAccountActivitiesRequest.Category

[gcatlin]

For example:

type AccountActivityCategory string

const (
    TradeActivity    AccountActivityCategory = "trade_activity"
    NonTradeActivity AccountActivityCategory = "non_trade_activity"
)

type GetAccountActivitiesRequest struct {
	ActivityTypes []string  `json:"activity_types"`
	Date          time.Time `json:"date"`
	Until         time.Time `json:"until"`
	After         time.Time `json:"after"`
	Direction     string    `json:"direction"`
	PageSize      int       `json:"page_size"`
	PageToken     string    `json:"page_token"`
	Category      AccountActivityCategory    `json:"category"`  // changed from type `string`
}

GetAccountActivitiesRequest
API Reference: Retrieve Account Activities