Plugin install endpoint: slug should never be empty

the plugin slug could have been empty if the plugin was a single file,
and not stored in a directory
@roccotripaldi have you tried passing directly the plugin filename?

This kind of request seems to work:
http://api.wordpress.org/plugins/info/1.0/jetpack/jetpack
http://api.wordpress.org/plugins/info/1.0/jetpack/jetpack.php