Mail's implementations of equals and hashCode are incomplete #718

Issue Summary

The implementations of Mail#equals and Mail#hashCode are missing fields. Two instances will be considered equal despite having different values in from, to, content, etc. Having a distinct hash code is not a requirement, but could lead to poor performance in some cases.

Steps to Reproduce

1. Create a first Mail instance as m1.
2. Create a second Mail instance as 'm2' with the same subject, but different from, to, and content.
3. Find that m1.equals(m2) is true despite differing in most set fields.

Code Snippet

final var subject = "Hello";
final var m1 = new Mail(
        new Email("a@mail.invalid"),
        subject,
        new Email("b@mail.invalid"),
        new Content(MediaType.TEXT_PLAIN, "Foo"));
final var m2 = new Mail(
        new Email("x@mail.invalid"),
        subject,
        new Email("y@mail.invalid"),
        new Content(MediaType.TEXT_PLAIN, "Bar"));

System.out.println(m1.equals(m2));
System.out.println(m1.hashCode());
System.out.println(m2.hashCode());

Prints

true
1961385645
1961385645

Technical details:

• sendgrid-java version: 4.8.3
• java version: 17.0.1 (this what I am using, but this issue is present in all versions)