Fix DefaultExchangeRate's equals and hashCode methods

[sgrimm-sg]

This changes DefaultExchangeRate to fix two related issues, #145 and #146.
The two are tied together because x.equals(y) should imply (and require)
x.hashCode() == y.hashCode().

[sgrimm-sg]

If you just want the fix for the infinite recursion in hashCode() but not the behavior change in equals() I'll be happy to make a separate PR just for that change.

[otaviojava]

Thanks.

[keilw]

Is both Moneta and Moneta-BP addressed in the same way?;-) IMHO maybe it would even require additional TCK or other tests in both implementations.

[sgrimm-sg]

I'm happy to cherry-pick my changes onto BP and make a new pull request if the maintainers would like. I don't know what the normal process for keeping BP in sync is; it seems to have a very different-looking commit history from the main repo. I suppose I sort of assumed things would get pulled over there automatically when a new release was tagged, but my bad for not asking.

[keilw]

It would make it easier and consistent and allow you to test against both if the behavior changed the same way.

[sgrimm-sg]

All right, PRs submitted. Thanks for pointing out I missed this; if I submit other PRs in the future I'll be sure to update both versions.