Update wordpressdotcom.rb

When invoked from the command line using `jekyll import wordpressdotcom -s $FILENAME`, the `filename` argument being passed in to WordpressDotCom::process() is not a simple string. It is instead a Hash, and as such the method's dying on line 15. This patch fixes the error.
Fixed fall-through behavior in method definition.

My previous commit fixed a bug in how the method handles the `filename` argument, but introduced a new one in that I hadn't updated the method's default `filename` value to _also_ be a hash. This commit fixes that.
Merge pull request #24 from stevenbedrick/patch-1

Update wordpressdotcom.rb to use its method parameter correctly.