Add a few client tests

[jussih]

I noticed there are hardly any tests for the client code. I was thinking about how to test genservers in Elixir and this is what I came up with. I call handle_data and expect a message to the handler process, which is actually the ExUnit process itself.

I'm open for discussion. Does this make any sense? Are there better conventions for testing in Elixir?

[coveralls]

Coverage increased (+3.4%) to 34.267% when pulling 7b08539 on jussih:client-test into 295a72d on bitwalker:master.

[coveralls]

Coverage increased (+3.4%) to 34.267% when pulling 7b08539 on jussih:client-test into 295a72d on bitwalker:master.

[coveralls]

Coverage increased (+3.4%) to 34.267% when pulling 7b08539 on jussih:client-test into 295a72d on bitwalker:master.

[bitwalker]

This works for the Client module, but doesn't work in general for testing GenServers, the usual approach is to start the process, and send messages/expect replies or examine the server state:

me = self()
{:ok, pid} = Agent.start_link(fn -> 1 end)
Agent.update(pid, fn n -> n + 1 end)
Agent.update(pid, fn n -> send(me, {:n, n}); n end)
assert 2 = :sys.get_state(pid)
assert_receive {:n, 2}

The above example is trivial, but hopefully illustrates the point. Agent is built on GenServer, and we're doing two things: using :sys.get_state/1 to get the internal state of the server so we can examine it for the test, and asserting on receiving a message. The latter can be useful when you expect a server to send a response to a caller, which in tests is going to be the test process.

For this PR, it is not necessary to do that, but in general that's the usual approach :)