Insert newline between base64 value and next multipart separator

[niklasl]

In my tests with IE7 on Win XP, a newline is needed between a base64 value and the following next multipart separator. The simplest fix seems to be to change Line 235 in CSSURLEmbedder to (234-235 shown):

                        //create MHTML header entry
                        mhtmlHeader.append("\n--");

(N.B. I haven't tested this code change, my local "fix" just rewrites the output to basically the same effect.)