REST: Fixed issues with rest POST method

Fixes the below issues in rest actions
 - Fetching dbus signature from xml arg elements.
 - Double iterator increament, while reading array
   of strings 'as'.

Test:
Verified rest API's with actions(POST).

Change-Id: I52ca938bae54c882da2f9f445ea0b4e605a5f8c3
Signed-off-by: AppaRao Puli <apparao.puli@linux.intel.com>