[gatsby-medium-source] convert timestamps to ISO 8601 date string (#2099)
[gatsby-medium-source] convert timestamps to ISO 8601 date string (#2099)

* serialize big integers for gatsby-medium-source timestamps

* typos

* typo

* convert timestamps to  ISO 8601 date string

Khaledgarbaya/gatsby,fabrictech/gatsby,danielfarrell/gatsby,mingaldrichgan/gatsby,mingaldrichgan/gatsby,Khaledgarbaya/gatsby,mickeyreiss/gatsby,fk/gatsby,mickeyreiss/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,Khaledgarbaya/gatsby,danielfarrell/gatsby,0x80/gatsby,0x80/gatsby,gatsbyjs/gatsby,danielfarrell/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,ChristopherBiscardi/gatsby,fabrictech/gatsby,gatsbyjs/gatsby,mickeyreiss/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,fk/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,fk/gatsby,ChristopherBiscardi/gatsby