feat: add send_each_at header parameter

examples/Example.java

@@ -68,6 +68,9 @@ public static void main(String[] args) {
     // [Scheduling](https://sendgrid.com/docs/API_Reference/SMTP_API/scheduling_parameters.html)
     header.setSendAt(1416427645);
 
+    // [Per email scheduling](https://docs.sendgrid.com/for-developers/sending-email/scheduling-parameters)
+    header.addSendEachAt(1416427645);
+
     //int sendAt = header.getSendAt();
 
     // Get Headers

src/main/java/com/sendgrid/smtpapi/SMTPAPI.java

@@ -2,6 +2,7 @@
 
 
 import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
 import org.json.JSONArray;
 import org.json.JSONException;
@@ -203,7 +204,23 @@ public SMTPAPI setSendAt(int sendAt) throws JSONException {
 
   public int getSendAt() throws JSONException {
     return this.header.getInt("send_at");
+  }
+
+  public SMTPAPI addSendEachAt(int sendAt) throws JSONException {
+    if (!this.header.has("send_each_at")) {
+      this.header.put("send_each_at", new JSONArray());
+    }
+    this.header.accumulate("send_each_at", sendAt);
+    return this;
+  }
 
+  public List<Integer> getSendEachAt() throws JSONException {
+    JSONArray array = this.header.getJSONArray("send_each_at");
+    List<Integer> sendTimes = new ArrayList<>();
+    for (int i = 0; i < array.length(); i++) {
+      sendTimes.add(array.getInt(i));
+    }
+    return sendTimes;
   }
 
   // convert from string to code point array

src/test/java/com/sendgrid/smtpapi/SMTPAPITest.java

@@ -12,6 +12,7 @@
 
 import java.io.*;
 import java.util.Calendar;
+import java.util.List;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
@@ -151,6 +152,14 @@ public class SMTPAPITest {
     Assert.assertEquals(expected, test.getSendAt());
   }
 
+  @Test public void testSetSendEachAt() throws JSONException {
+    int expected = 12345;
+    test.addSendEachAt(expected);
+    List<Integer> output = test.getSendEachAt();
+    Assert.assertEquals(1, output.size());
+    Assert.assertEquals(expected, test.getSendEachAt().get(0), 0);
+  }
+
   @Test public void testCopyrightDateRange() throws JSONException {
     int expectedYear = Calendar.getInstance().get(Calendar.YEAR);
     String copyRightLine = getCopyrightDateRangeLine("LICENSE");