Specify that domain should be nil if MAILGUN_SMTP_LOGIN not set

README.md

@@ -51,10 +51,9 @@ you can instead use your Mailgun public key to authenticate like this:
 ```ruby
 require 'mailgunner'
 
-mailgun = Mailgunner::Client.new({
-  api_key: 'pubkey-5ogiflzbnjrljiky49qxsiozqef5jxp7',
-  domain: nil
-})
+public_key = 'pubkey-5ogiflzbnjrljiky49qxsiozqef5jxp7'
+
+mailgun = Mailgunner::Client.new(api_key: public_key)
 
 response = mailgun.validate_address('john@gmail.com')
 ```

lib/mailgunner.rb

@@ -8,7 +8,7 @@ class Client
     attr_accessor :domain, :api_key, :http
 
     def initialize(options = {})
-      @domain = options.fetch(:domain) { ENV.fetch('MAILGUN_SMTP_LOGIN').split('@').last }
+      @domain = options.fetch(:domain) { ENV['MAILGUN_SMTP_LOGIN'].to_s.split('@').last }
 
       @api_key = options.fetch(:api_key) { ENV.fetch('MAILGUN_API_KEY') }
 

spec/mailgunner_spec.rb

@@ -51,6 +51,12 @@ def expect(request_class, arg)
 
       Mailgunner::Client.new(api_key: @api_key).domain.must_equal(@domain)
     end
+
+    it 'defaults to nil if the MAILGUN_SMTP_LOGIN environment variable does not exist' do
+      ENV.delete('MAILGUN_SMTP_LOGIN')
+
+      Mailgunner::Client.new(api_key: @api_key).domain.must_be_nil
+    end
   end
 
   describe 'api_key method' do