Users API: Fixed sending invite when using form requests

- Cast send_invite value in cases where it might not have been a boolean,
  which occurs on non-JSON requests.
- Added test to cover.
- Updated API docs to mention and shown boolean usage.

tests/Api/UsersApiTest.php

@@ -143,6 +143,23 @@ class UsersApiTest extends TestCase
         Notification::assertSentTo($user, UserInviteNotification::class);
     }
 
+    public function test_create_with_send_invite_works_with_value_of_1()
+    {
+        $this->actingAsApiAdmin();
+        Notification::fake();
+
+        $resp = $this->postJson($this->baseEndpoint, [
+            'name'        => 'Benny Boris',
+            'email'       => 'bboris@example.com',
+            'send_invite' => '1', // Submissions via x-www-form-urlencoded/form-data may use 1 instead of boolean
+        ]);
+
+        $resp->assertStatus(200);
+        /** @var User $user */
+        $user = User::query()->where('email', '=', 'bboris@example.com')->first();
+        Notification::assertSentTo($user, UserInviteNotification::class);
+    }
+
     public function test_create_name_and_email_validation()
     {
         $this->actingAsApiAdmin();