Wrong 'Created' field for Certificates

[whachoe]

Issue summary

Created-timestamps for certificates are shown as Jan 18, 1970.

I verified that they are coming in correctly via the /certificates call in dashboard.
Example of a recent one:

"created_at": 1547203708 (which translates to "Fri, 11 Jan 2019 10:48:28 GMT")

Your environment

• Kong version = 0.14.1
• Kong Dashboard version = 3.5.0 (Kong Dashboard versions before 3.0.0 aren't supported anymore)
• Node version = #.#.#
• Npm version = #.#.#

Issue description

Current behavior

All created-dates are set to "Jan 18, 1970".

Expected behavior

Correct date should be displayed

Steps to reproduce

Click on the 'Certificates' tab.

Good to have

Any screenshots? Browsers' logs?
Screenshot: https://cl.ly/342dfdc3d436