Content tag parsing fix

Content tag parsing fix

[Issue#]       LINUXWRT-438
[Bug]          Bug in content parsing when src == "" is treated as if not encountered at all
[Cause]        Wrong content parswing changes
[Solution]     Fix
[Verification] Run tests:
 wrt-extra-tests-w3c --output=text --regexp='br'
 wrt-extra-tests-w3c --output=text --regexp='bq'
 wrt-extra-tests-w3c --output=text --regexp='bs'
 wrt-extra-tests-w3c --output=text --regexp='d7'
 wrt-extra-tests-w3c --output=text --regexp='d8'
 wrt-extra-tests-w3c --output=text --regexp='gb'
 wrt-extra-tests-w3c --output=text --regexp='b3'
 wrt-extra-tests-misc --output=text --regexp='first_content'
 wrt-extra-tests-misc --output=text --regexp='prefer_tizen_content'

Change-Id: I4d0eb996a4bcebbfab960fbd73c41eb04c124dde