ImageLoader: setError after null check

if request was canceled it will have already been removed
from mInFlightRequests and cause NPE

Change-Id: I1ce78bb6b3b2964545a47b5e3bb59bdf1a6d940e
Uses the "Last-Modified" header for "If-Modified-Since"
see http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.4
Updates the cache headers of a cache entity in case of HTTP 304
see http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
Update only cache headers in case of HTTP 304
Merge "ImageLoader: setError after null check"
Merge branch 'master' of https://android.googlesource.com/platform/frameworks/volley
check for empty response headers
adds OnFinishListener