fix(python sdk): Fix timestamps & context properties

[ehdsouza]

As quoted in issue in the swagger:

We had the created & updated properties still marked as required.
When we were merging the request & response models for Context, we left out the conversation_id and system to be required which is not true when in a request model but is valid on response model.
This fixes the above to two issues by making the properties to be optional so that the SDK's don't break.

[anweshan]

You should regenerate assistant with what is on master, because now while you committed changes to conversation with digress_in etc, it's not in assistant

[anweshan]

It has digress_in so it is good

[codecov-io]

Codecov Report

Merging #413 into develop will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           develop    #413   +/-   ##
=======================================
  Coverage        0%      0%           
=======================================
  Files           22      22           
  Lines        12997   12971   -26     
=======================================
+ Misses       12997   12971   -26

Impacted Files	Coverage Δ
watson_developer_cloud/assistant_v1.py	0% <0%> (ø)	⬆️

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