retry method: Handle dates given in `Retry-After` headers

[sckott]

via #95

this shouldn't happen very often but can happen apparently

as.numeric() does not fail, but instead throws a warning.

as.numeric('2018-12-05T00:00:00.000Z')
[1] NA
Warning message:
NAs introduced by coercion

we can tryCatch this perhaps or look for a date in the string and handle accordingly

[sckott]

removing v0.8 milestone for now - will incorporate a test once we have a real world use case of this