Fix emptystring filename header

I was parsing some youtube pages and got into this issue that sometimes the response object for the Parser looks like this:

  {"access-control-allow-origin"=>"*",
 "etag"=>"\"v0\"",
 "expires"=>"Sun, 09 Jun 2013 22:53:27 GMT",
 "cache-control"=>"public, max-age=86400, no-transform",
 "content-disposition"=>"inline;filename=\"\"",
 "content-type"=>"image/png",
 "x-content-type-options"=>"nosniff",
 "date"=>"Sat, 08 Jun 2013 22:53:27 GMT",
 "server"=>"fife",
 "content-length"=>"1667",
 "x-xss-protection"=>"1; mode=block"} 

This results an exception inside .extract_filename for the parser object.
 
I've added a test case as well, so you can see that it failes with the current implementation. Not sure if this is the right solution but for me it works.