Create valid path if no api.format specified

Instagrams 

    /media/shortcode/{shortcode}

endpoint does not currently respond to requests which include a requested return format, ie

    /media/shortcode/D.json

and thus leave the api.media_shortcode(shortcode) permanently broken as the _build_path() will always append a trailing "."

    /media/shortcode/D.

is also invalid.

Code change removes trailing "." if api.format = '' allowing the following valid request to be made
    
    /media/shortcode/D


Is it really necessary to specify a format as instagrams own docs state [endpoint responses are returned as json](http://instagram.com/developer/endpoints/)