fix(skytrace): send parsed json body correctly to expect plugin

[bernardobridge]

Context

The Skytrace Smoke tests have been failing, due to a regression introduced by: 8a3e5c2

In reality, that logic is fine, but Skytrace was sending the body as a string when it should be an object (JSON).

This PR does the following:

• Checks for contentType using test instead of match instead, so a boolean is returned (rather than null) when there is no match
• If it's a JSON, parses it to an object, otherwise sends content as is.

Testing

The following commands were used to test this:

(the failing test):

./node_modules/skytrace/bin/run ping TEST_URL/api/load-tests/:uuid \
          -H "X-Auth-Token: API_KEY_HERE" \
          -bp \
          -e "statusCode: 200" \
          -e "contentType: application/json; charset=utf-8" \
          -e "jmespath: report.aggregate.summaries.[\"browser.page.FCP.https://www.artillery.io/docs\"][0].max != null"

./node_modules/skytrace/bin/run ping TEST_URL/api/user/whoami \                                             
          -H "X-Auth-Token: API_KEY_HERE" \
          -bp \
          -e "statusCode: 200" \
          -e "contentType: application/json; charset=utf-8"

./node_modules/skytrace/bin/run http post TEST_URL/api/load-tests/:uuid/note \
          -H "X-Auth-Token: API_KEY_HERE" \
          --json "{text: another note from skytrace}" \
          -e "statusCode: 204"