Declare parameter with default to match signature, avoid fatal error.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@871994 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme with bugfix info and link, version bump to 0.7.8.1.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@871995 b8457f37-d9ea-0310-8a92-e5e31aec5664