Update Http.php

When $json is a string, the condition on line 48 is evaluated like so :
isset($json['filename'])
-> isset($json[0])  (string when interpreted as a integer becomes 0)
-> isset({first character of $json})

And so fopen then attempts to open a file named '{'.
Merge pull request #20 from rene-lafleur/patch-1

Update Http.php