parameterize retrying-on-value

[elennick]

https://github.com/elennick/retry4j/blob/master/src/main/java/com/evanlennick/retry4j/config/RetryConfig.java#L20

Retrying by value doesn't need to just use a vague Object like this... we know what the type is. Can use generics to more strongly type this to whatever the return value of the Callable is.

[dev3loperb]

Can you share the status of this issue, please. I've seen that there was the PR #64, but it's closed.
I'd like to extend this to accept null values. Maybe it will be needed to add a new method aka retryOnNull, not sure yet.

[elennick]

@ipergenitsa I started investigating resolving it a while back but wasn't happy with the direction my solution was taking so I closed that PR. If you'd like to take another stab at it and submit a PR that would be totally fine... i can assign this to you for now to look into.