Fix LocalAdapter.deliver/2 return type

[SamuelWillis]

Fixes issue #588

Fix the LocalAdapter.deliver/2 to return a {:ok, result} tuple when
the adapter is configured with the open_email_in_browser_url option.

This does not include a test as I could not come up with a way to test the local adapter with out it attempting a System.cmd call and trying to open a file/browser window while running the tests.

Any suggestions there would be appreciated.

I was looking at adding the test to the Bamboo.Adapters.LocalAdapterTest like the following:

  test "sent emails has emails that were delivered synchronously" do
    email = new_email(subject: "This is my email")

    config = %{
      open_email_in_browser_url: ""
    }

    {:ok, _response} = email |> LocalAdapter.deliver(config)

    assert [%Bamboo.Email{subject: "This is my email"}] = SentEmail.all()
  end

The above results in a message being logged while running the tests: "The file $filename does not exist."

[germsvel]

Thanks for opening this PR @SamuelWillis. I think there might be a good way to test this — probably something like https://github.com/thoughtbot/bamboo/pull/590/files has (a similar PR for this same issue) — but I'm okay getting this in as is, and I'll see if we can follow up with a test in #590.