add datetime representer and allow unicode characters

[KatKatKateryna]

Overview

This PR is fixing the behavior of overwriting example-config.yaml via admin API. Currently, the data is received correctly but YAML writer is representing datetime objects and special characters incorrectly:

To fix this, YAML writer now has 'allow_unicode=True' and 'Dumper=MyDumper', where MyDumper is an instance of yaml.SafeDumper with correct datetime object representers.

Related Issue / discussion

This was discovered while working on QGIS pygeoapi-config plugin: correctly sent data is being saved incorrectly on the running pygeoapi instance: byteroad/pygeoapi_config#12

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