timestamps in stix2 MUST be formatted as UTC

[pbedat]

When using Timestamps with a location other than UTC, the library currently marshals them "as is"

e.g. "2022-01-02T00:00:00+01:00" instead of "2022-01-01T23:00:00Z"

This change converts the timestamp to UTC before marshalling

[codecov[bot]]

Codecov Report

Merging #57 (0fd3d59) into master (98d3b62) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   99.30%   99.30%           
=======================================
  Files          44       44           
  Lines        2009     2009           
=======================================
  Hits         1995     1995           
  Misses          7        7           
  Partials        7        7           

Impacted Files	Coverage Δ
data-types.go	100.00% <100.00%> (ø)

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