Allow sending emails with just text or just html bodies. Fixes #14

spec/carbon_smtp_adapter_spec.cr

@@ -30,6 +30,13 @@ class TestEmail < BaseEmail
   header "Sender", "support@myapp.com"
 end
 
+class NoHtmlEmail < BaseEmail
+  from Carbon::Address.new("My App Name", "support@myapp.com")
+  to "fred@example.org"
+  subject "NoHtml Subject"
+  templates text
+end
+
 describe CarbonSmtpAdapter do
   it "works" do
     email = TestEmail.new
@@ -44,4 +51,11 @@ describe CarbonSmtpAdapter do
     received_email.should match(/Content-Type: text\/html/)
     received_email.should match(/X-Crystal-Version: 0\.27/)
   end
+
+  it "sends with just text template" do
+    email = NoHtmlEmail.new
+    delivered = Carbon::SmtpAdapter.new.deliver_now(email)
+
+    delivered.should_not eq(nil)
+  end
 end

spec/templates/no_html_email/text.ecr

@@ -0,0 +1 @@
+hi

src/carbon/adapters/smtp_adapter.cr

@@ -44,8 +44,13 @@ class Carbon::SmtpAdapter < Carbon::Adapter
         end
       end
 
-      message email.text_body
-      message_html email.html_body
+      if text = email.text_body.presence
+        message(text)
+      end
+
+      if html = email.html_body.presence
+        message_html(html)
+      end
     end
   end