Add test for mime parsing reportedly crash-inducing message

.gitattributes

@@ -2,6 +2,10 @@
 # ensures this even if the user has not set core.autocrlf.
 *  text=auto
 
+# This directory contains email messages verbatim, and changing CRLF to
+# LF will corrupt them.
+test-data/* text=false
+
 # binary files should be detected by git, however, to be sure, you can add them here explicitly
 *.png  binary
 *.jpg  binary