Update base64 encoding to return UTF-8 string

[francescoingv]

Overview

Fixed a simple error in json_serial() not returning a string: Decode base64 encoded JSON object to UTF-8 string.

Related Issue / discussion

No previous issue: just a code fix.
Difficult to provide a test.

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)

• [ X] 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
• [ X] I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[tomkralidis]

@francescoingv we need an issue and a test to demonstrate the issue in order to consider this PR.