Fixes issue #1200; instead of using about:blank as an error proxy for clear.cache.gif in ClippedImageImplIE6.java, uses "http://" or "https://" (depending on the host page base URL).

Found by: rustyshelf
Patch by: rdayal, scottb
Review by: scottb


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1188 8db76d5a-ed1c-0410-87a9-c151d255dfc7