mi_json: escape '\\' character

also fix incorrect '"' escaping

(cherry picked from commit 28b399bb166a786bb0e943ff072606d2d01d96b6)

Conflicts:
	modules/mi_json/http_fnc.c
remove bogus flushing for mi_json

TODO: find a proper way for handling flushing, since the current
implementation leads to a infinite loop

(cherry picked from commit 0d47a32d18c656a22aa536f5eeb24e3310ade481)