Fix q=0 crash and ordering in get_choice_from_headers

[Shubham-Padkonde]

• A header entry with q=0 (e.g. "Accept-Language: fr;q=0, en") raised ZeroDivisionError from 1 / q_value. q=0 means "not acceptable", so such entries are now skipped; if nothing acceptable remains, None is returned as for a missing header.
• With all=True the choices were returned in heap order rather than sorted order, e.g. "a;q=0.1,b;q=0.2,c;q=0.3,d;q=0.4,e;q=0.9" gave ['e', 'd', 'b', 'a', 'c']. Choices are now sorted by descending q, keeping header order for ties.
• An unparsable q value such as "q=1.2.3" is skipped instead of raising.

Overview

Related Issue / discussion

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