Fix #404 Add ability to impersonate subusers

src/main/java/com/sendgrid/SendGrid.java

@@ -41,6 +41,9 @@ public class SendGrid implements SendGridAPI {
   /** The number of milliseconds to sleep between retries. */
   private int rateLimitSleep;
 
+  /** The subuser to be impersonated. */
+  private String subuser;
+
   /**
    * Construct a new SendGrid API wrapper.
    * @param apiKey is your SendGrid API Key: https://app.sendgrid.com/settings/api_keys
@@ -193,6 +196,31 @@ public void setRateLimitSleep(int rateLimitSleep) {
     this.rateLimitSleep = rateLimitSleep;
   }
 
+  /**
+   * Impersonate subuser for subsequent requests
+   * @param subuser the subuser to be impersonated
+   */
+  public void addImpersonateSubuser(String subuser) {
+    this.subuser = subuser;
+    this.addRequestHeader("on-behalf-of", subuser);
+  }
+
+  /**
+   * Stop Impersonating the subuser
+   */
+  public void removeImpersonateSubuser() {
+    this.subuser = null;
+    this.removeRequestHeader("on-behalf-of");
+  }
+
+  /**
+   * Get the impersonated subuser or null if empty
+   * @return the impersonated subuser
+   */
+  public String getImpersonateSubuser() {
+    return this.subuser; 
+  }
+
   /**
    * Makes the call to the SendGrid API, override this method for testing.
    * @param request the request to make.

src/test/java/com/sendgrid/SendGridTest.java

@@ -3406,4 +3406,33 @@ public void test_whitelabel_links__link_id__subuser_post() throws IOException {
     Assert.assertEquals(200, response.getStatusCode());
   }
 
+  @Test
+  public void test_add_impersonate_subuser() {
+    SendGrid sg = new SendGrid(SENDGRID_API_KEY);
+
+    sg.addImpersonateSubuser("subusername");
+    Assert.assertEquals(sg.getRequestHeaders().get("on-behalf-of"), "subusername");
+  }
+
+  @Test
+  public void test_remove_impersonate_subuser() {
+    SendGrid sg = new SendGrid(SENDGRID_API_KEY);
+
+    sg.addImpersonateSubuser("subusername");
+    Assert.assertEquals(sg.getRequestHeaders().get("on-behalf-of"), "subusername");
+
+    sg.removeImpersonateSubuser();
+    Assert.assertEquals(sg.getRequestHeaders().get("on-behalf-of"), null);
+  }
+
+  @Test
+  public void test_get_impersonate_subuser() {
+    SendGrid sg = new SendGrid(SENDGRID_API_KEY);
+
+    sg.addImpersonateSubuser("subusername");
+    Assert.assertEquals(sg.getImpersonateSubuser(), "subusername");
+
+    sg.removeImpersonateSubuser();
+    Assert.assertEquals(sg.getImpersonateSubuser(), null);
+   }
 }