Use custom encoder (if provided) for all view `key` params not just `keys`

[mojito317]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md) or test/build only changes
• [x] Completed the PR template below:

Description

Fixes https://github.com/cloudant/python-cloudant/issues/221#issuecomment-752424499

Approach

Used custom encoder at _py_to_couch_translate for endkey, key, and startkey parameters.

Schema & API Changes

endkey, key and startkey parameters will be encoded with the custom encoder.

Security and Privacy

• No change

Testing

Test that all_docs uses the custom encoder. Fixed test_valid_endkey, test_valid_key and test_valid_startkey with long type.

Monitoring and Logging

• No change

[ricellis]

Still needs a changes entry https://github.com/cloudant/python-cloudant/pull/501#pullrequestreview-676251324

but otherwise I'm ok with this

[mojito317]

Still needs a changes entry #501 (review)

but otherwise I'm ok with this

Right, I added it in ab304b2.

[mojito317]

Thanks for the review @emlaver and @ricellis! I'm on it to merge this.