reformatted lib/exe/fetch.php
run main code only for non-tests in lib/exe/fetch.php
better HTTPClient debug output on CLI
added another HTTPClient test
HTTPClient: fixed max_bodysize when using keep-alive
HTTPClient: updated function comments
HTTPclient: print number of received bytes on error
test for media downloading (related to FS#2675
don't use keep-alive with single shot HTTP clients