Better optional parameter integration

Apparently I'm dumb and didn't know about equal signs in parameters.

lib/twitter_ebooks/bot.rb

@@ -109,12 +109,8 @@ module Ebooks
     # Get an array of media uris in tweet.
     # @param size [String] A twitter image size to return. Supported sizes are thumb, small, medium (default), large
     # @return [Array<String>] image URIs included in tweet
-    def media_uris(*args)
-      # When there's no size given, the default is medium.
-      size = ''
-      # Was one given?
-      unless args.empty?
-        case args[0]
+    def media_uris(size_input = '')
+      case size_input
       when 'thumb'
         size = ':thumb'
       when 'small'
@@ -123,7 +119,8 @@ module Ebooks
         size = ':medium'
       when 'large'
         size = ':large'
-        end
+      else
+        size = ''
       end
 
       # Start collecting uris.