Make all limit parameters in Mastodon methods nullable with null default

[bocops]

As discussed in #293, all endpoints that accept a limit parameter coerce this value to be within the accepted range and do not fail for values outside that range, so no client-side checking of this value is necessary and might even turn out to be problematic if the accepted range ever changes.

As pointed out in #360, having default values for our limit parameters can be equally problematic. Instead, all limit parameters should default to null and only be added to the request if non-null. This ensures that the server's default value will always be used, unless specified by the library user.

[bocops]

Info: I just tested server behaviour by removing the limit from parameter lists built in several of our methods and then running relevant samples we provide. All of those I tested do work as expected even if no value for limit is provided.