HTTPJSONHandler http POST customisable header

[Lewih]

reframe/reframe/core/logging.py

Lines 605 to 609 in 088207c

	requests.post(
	self._url, data=json_record,
	headers={'Content-type': 'application/json',
	'Accept-Charset': 'UTF-8'}
	)

It would be nice to be able to customise the header of the http POST request via config file.
This may be useful for some services requiring auth via API Tokens while using the httpjson handler.