returnType not exposed, so can't set custom lifespan on cache items

[paullryan]

I'm submitting a ... (check one with "x")

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  <!-- Please check the repository for a similar issue or PR before submitting -->
[ ] Support request => <!-- Please check the repository for a similar issue or PR before submitting -->
[ ] Feature request
[ ] Documentation issue or request

Current behavior
The ReturnType model is not exposed to index.ts so can't be directly used by a calling model. Given that ReturnType comes before lifespan in CacheService.set this makes it so a user can't set the lifespan without mocking the ReturnType

Expected/desired behavior
ReturnType should be exposed and/or the lifespan definition should come before the returnType in CacheService.set so that lifespan is accessible.

Minimal reproduction of the problem with instructions
No reproduction needed as this a bug in the public api

What is the motivation / use case for changing the behavior?
Need the ability to use CacheService.set with a custom lifespan for short lived items.

Environment

• Angular version: 5.x

• Browser:
  N/A

• For Tooling issues:
  N/A

• Others:
  Note while I'm aware I can hack around this, the correct behavior would be to allow the developer access to the ReturnType model.

[fulls1z3]

@paullryan sorry for the delay, had busy times. feel free to fire a PR and i'll approve.