Relax detection of images for strip detection

It is perfectectly fine to have an image cited in an HTML body similar to:

<img width=909 height=302 src=cid:image002.jpg@01CF5426.BF5A72A0 alt=image>

Which may or may not have quoted @src attribute, and may very well have an
at sign (@) somewhere in the attribute text. The previous regular expression
would not match such a @src attribute.
Merge pull request #772 from greezybacon/issue/relax-stripped-cid-detect

Reviewed-By: Peter Rotich <peter@osticket.com>