Handle conversions for naive datetime values

[mkistler]

This PR fixes the datetime_to_string and string_to_datetime handling of dates without timezone information.

Python datetime values come in two forms: "naive" and "aware"

https://docs.python.org/3/library/datetime.html#aware-and-naive-objects

The fundamental difference between these two forms is that "naive" datetimes do not have a timezone, whereas "aware" datetimes do.

This PR fixes the core utility methods for converting datetime to/from strings.

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   97.78%   97.81%   +0.02%     
==========================================
  Files          17       17              
  Lines         588      594       +6     
==========================================
+ Hits          575      581       +6     
  Misses         13       13

Impacted Files	Coverage Δ
ibm_cloud_sdk_core/utils.py	99.16% <100%> (+0.04%)	⬆️

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[padamstx]

👍

[ibm-devx-automation]

🎉 This PR is included in version 1.6.2 🎉

The release is available on GitHub release

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