Fix payload normalization in http.request events. Closes #434

[mthenw]

What did you implement:

Closes #434

How did you implement it:

When we moved from HTTP event to http.request event as CloudEvent the normalization need to happen also for the Body in the event

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[codecov]

Codecov Report

Merging #459 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #459     +/-   ##
=========================================
+ Coverage   66.06%   66.17%   +0.1%     
=========================================
  Files          32       32             
  Lines        1877     1883      +6     
=========================================
+ Hits         1240     1246      +6     
  Misses        585      585             
  Partials       52       52

Impacted Files	Coverage Δ
event/http.go	100% <100%> (ø)	⬆️
event/event.go	78.12% <100%> (+0.7%)	⬆️

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