Use Content-Type application/cloudevents+json for http provider

[alexdebrie]

What did you implement:

For the http function provider, I changed the Content-Type of the outgoing request to application/cloudevents+json. This is in accordance with the CloudEvents spec.

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 #415 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   63.85%   63.85%           
=======================================
  Files          29       29           
  Lines        1649     1649           
=======================================
  Hits         1053     1053           
  Misses        553      553           
  Partials       43       43

Impacted Files	Coverage Δ
providers/http/http.go	70% <100%> (ø)	⬆️

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