typespec errors when using Erlang/Json encoder

[rwngallego]

There's a type mismatch when using an encoder like Json or Erlang as the typespecs for set/add/replace are defined as binary() instead of term(). For example, in the unit tests:

assert {:ok} == Memcache.set(pid, "hello", ["list", 1])

Conflicts with the set/4 definition of binary in the third parameter:

  @doc """
  Sets the key to value
  Accepted options: `:cas`, `:ttl`
  """
  @spec set(GenServer.server(), binary, binary, Keyword.t()) :: store_result
  def set(server, key, value, opts \\ []) do
    ...

When calling dialyzer in an app that uses the Erlang encoder we get:

The call 'Elixir.Memcache':add
         (_name@1 :: atom(),
          _key@1 :: any(),
          _data@1 ::
              #{'ts' := 'nil' | integer(),
                'ttl' := 'nil' | integer(),
                'value' := _},
          [{'ttl', integer()}, ...]) breaks the contract
          ('Elixir.GenServer':server(),
          binary(),
          binary(),
          'Elixir.Keyword':t()) ->
             store_result()

[ananthakumaran]

value type needs to be changed to term. I guess other functions like set_cas, multi_set are also affected. Feel free to open a PR if you got time, otherwise, I will change it over the weekends.