WorkerArgs specs are too narrow

[rlipscombe]

In poolboy:child_spec/3, poolboy:start/2 and poolboy:start_link/2, WorkerArgs is specified as proplists:proplist(). For example:

-spec start_link(PoolArgs :: proplists:proplist(),
                 WorkerArgs:: proplists:proplist())
    -> start_ret().

However, this value is passed directly to the worker's start_link function, and it doesn't have to be a proplist.

This is causing dialyzer to issue warnings for my project.

[rlipscombe]

Duplicate of #95