Correction to Oracle.py _get_previous() SQL logic

[jwallace94]

Overview

On the item level of collections, the previous link was not behaving as expected and not sending the user to the previous record in the collection. This PR corrects the logic of the SQL used for the previous record so that it actually gets the previous record as expected.

Related Issue / discussion

https://gajira.atlassian.net/browse/DAT-1039

Additional information

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[tWangchen]

LGTM