[Fix #336] coercion from interval to period

[AlunHewinson]

If the sign of the @month and @day slots of the new period are opposite to each other, set @day to zero.
#336

It is my understanding that negative periods are legal but a period created from an interval should not have a negative month slot and positive days or vice versa.

First attempted in pull request #346 but there was an error in the test and it didn't handle NAs. Sorry if it fails the testthat tests; I need to learn how to understand and run those myself locally. Hope you take this attempted bugfix in good faith if it doesn't work; I won't try again if it fails.

[vspinu]

I don't understand why you are setting day to 0. If the day componnet is negative, it's negative for a reason or not? Something is going wrong with the logic in that function. You cannot just overwrite it with 0 and label as fixed.

I will try to have a look at it today.

(You don't need close an issue and open a new one each time you change a thing. Simply git push --force to the same branch.)

[vspinu]

Fixed in 0bbe8c1