Honour globally configured test_mode as set in global config (using Mailgun.configure)

[eflukx]

Since long time (see #253 and 646041b) Client uses the defaults configured using the Mailgun.configure block. Somehow it skips the setup of test_mode. As it can be set in the global configuration, this seems to be an inconsistency.

[eflukx]

Another possible (but api-breaking) change is to use named parameters in the initialize method. e.g.

    def initialize(api_key: Mailgun.api_key,
                   api_host: Mailgun.api_host || 'api.mailgun.net',
                   api_version: Mailgun.api_version  || 'v3',
                   ssl: true,
                   test_mode: !!Mailgun.test_mode,
                   timeout: nil,
                   proxy_url: Mailgun.proxy_url)