new `publicationDate` element

[wibeasley]

It looks like there is a new component returned from the dataverse_contents() function. I'll add it to the tests so they pass again

Error: actual[[1]] not equal to `expected`.
Names: 2 string mismatches
Length mismatch: comparison on first 8 components
Component 7: 1 string mismatch
Component 8: 1 string mismatch

expected  <- structure(
  list(
    id                = 396356L,
    identifier        = "FK2/FAN622",
    persistentUrl     = "https://doi.org/10.70122/FK2/FAN622",
    protocol          = "doi",
    authority         = "10.70122",
    publisher         = "Demo Dataverse",
    storageIdentifier = "file://10.70122/FK2/FAN622",
    type              = "dataset"
  ),
  class = "dataverse_dataset"
)

> actual[[1]]
Dataset (396356): https://doi.org/10.70122/FK2/FAN622
Publisher: Demo Dataverse
publicationDate: 2020-04-22

> expected
Dataset (396356): https://doi.org/10.70122/FK2/FAN622
Publisher: Demo Dataverse

edit: now I'm pretty sure this is related to "releasing"i it (ref #61)