func (h *SMTPAPIHeader) AddSection(section, value string) {
	if h.Section == nil {
		h.Section = make(map[string]string)
	}
	h.Section[section] = value
}
AddSection sets the value for a specific section
package smtpapi
import (
	"bytes"
	"encoding/json"
	"fmt"
	"unicode/utf16"
)
const Version = "0.6.6"
type SMTPAPIHeader struct {
	To          []string               `json:"to,omitempty"`
	Sub         map[string][]string    `json:"sub,omitempty"`
	Section     map[string]string      `json:"section,omitempty"`
	Category    []string               `json:"category,omitempty"`
	UniqueArgs  map[string]string      `json:"unique_args,omitempty"`
	Filters     map[string]Filter      `json:"filters,omitempty"`
	ASMGroupID  int                    `json:"asm_group_id,omitempty"`
	ASMGroups   []int                  `json:"asm_groups_to_display,omitempty"`
	SendAt      int64                  `json:"send_at,omitempty"`
	SendEachAt  []int64                `json:"send_each_at,omitempty"`
	IpPool      string                 `json:"ip_pool,omitempty"`
	BatchID     string                 `json:"batch_id,omitempty"`
	DynamicData map[string]interface{} `json:"dynamic_template_data,omitempty"`
}
type Filter struct {
	Settings map[string]interface{} `json:"settings,omitempty"`
}
func NewSMTPAPIHeader() *SMTPAPIHeader {
	return &SMTPAPIHeader{}
}
func (h *SMTPAPIHeader) AddTo(email string) {
	h.To = append(h.To, email)
}
func (h *SMTPAPIHeader) AddTos(emails []string) {
	for i := 0; i < len(emails); i++ {
		h.AddTo(emails[i])
	}
}
func (h *SMTPAPIHeader) SetTos(emails []string) {
	h.To = emails
}
func (h *SMTPAPIHeader) AddSubstitution(key, sub string) {
	if h.Sub == nil {
		h.Sub = make(map[string][]string)
	}
	h.Sub[key] = append(h.Sub[key], sub)
}
func (h *SMTPAPIHeader) AddSubstitutions(key string, subs []string) {
	for i := 0; i < len(subs); i++ {
		h.AddSubstitution(key, subs[i])
	}
}
func (h *SMTPAPIHeader) SetSubstitutions(sub map[string][]string) {
	h.Sub = sub
}