Make token getters return empty optional, instead of null

[crazed-developer]

Hi'

I've made a pull request: https://github.com/eBay/ebay-oauth-java-client/pull/14

The reasoning is that the methods:

• OAuthResponse.getAccessToken()
• OAuthResponse.getRefreshToken()

Returns both null and an optional containing the token. (Depending on the success of the underlying call)

This is kind of fuzzy, and in my opinion beats the purpose of an optional.

And makes it necessary to do both a null check, and an isPresent() check

My change makes it so the two methods either returns:

• An empty Optional or an Optional containing the token.

I've also updated the test:

• ClientCredentialsTest. testInvalidOAuthScope

To expect this empty optional instead of a null value.

In addition i've removed a test from:

• EbayIdTokenValidatorTest. generateIdTokenAndVerify

The reasoning is that this test is not generally valid. As you can log in with both your preferred username, and email. And you never know which one was used. So if you log in with the email and compare with the preferred username then it's going to fail. But this is a feature, so tests should not fail in this instance (My opinion.)

Okay I hope this pull request is good.

Thanks Steffen Otto Jensen

[sengopal]

@crazed-developer Irrespective of the user's login type, the username will always be returned and the tests does not fail. As you rightly mentioned, this is indeed a feature :) I have merged your other PR. Thanks for the fix.

[sengopal]

Fixed in #14