Consider 200 response with "Content-length <= 4" to not be a captive portal.

There's no point in considering this a captive portal as the user cannot
sign-in to an empty page. Probably the result of a broken transparent proxy.

Bug: 122999481
Test: Manual

Change-Id: I1ca44092ac05a1708d9b41c5a4aad545b71b7eab
Signed-off-by: Sehee Park <sehee32.park@samsung.com>