The pagerduty action allows to send contexts, which contains an array
of texts or images, each with a link.
The field of this data was named 'context' instead of 'contexts' and
thus those contects were never correctly parsed on the pagerduty side.
Unfortunately pagerduty accepts any JSON, thus this was not caught so
far.
This commit allows parsing of the old field name to retain BWC, but when
written out via toXContent, it will always use the 'contexts' field name.
relates elastic/x-pack-elasticsearch#3184
Original commit: elastic/x-pack-elasticsearch@50f0b65d56
fdb02f4f99