GithubRequest.with() methods should not silently ignore null values

[bitwiseman]

The GithubRequest.with() currently discard null values. Only GithubRequest.withNullable() keeps them. This is convenient sometimes but leaves things open for confusing issues with missing fields. Also, the presence of withNullable() implies (via Spotbugs annotations) that the with() methods require Nonnull.

Consider adding a withOptional() that ignores null values and changing with() to not ignore null values.

See #899