Use the first registered connection if none are provided

spec/adapter/adapters_spec.cr

@@ -1,11 +1,15 @@
 require "../spec_helper"
 
 class Foo < Granite::Base
-  connection mysql
+  connection sqlite
 
   column id : Int64, primary: true
 end
 
+class Bar < Granite::Base
+  column id : Int64, primary: true
+end
+
 describe Granite::Connections do
   describe "registration" do
     it "should allow connections to be be saved and looked up" do
@@ -24,6 +28,12 @@ describe Granite::Connections do
 
     it "should assign the correct connections to a model" do
       adapter = Foo.adapter
+      adapter.name.should eq "sqlite"
+      adapter.url.should eq ENV["SQLITE_DATABASE_URL"]
+    end
+
+    it "should use the first registered connection if none are specified" do
+      adapter = Bar.adapter
       adapter.name.should eq "mysql"
       adapter.url.should eq ENV["MYSQL_DATABASE_URL"]
     end

src/granite/connections.cr

@@ -7,7 +7,7 @@ module Granite
       @@registered_connections << adapter
     end
 
-    def self.[](name : String) : Granite::Adapter::Base?
+    def self.[](name : String?) : Granite::Adapter::Base?
       registered_connections.find { |conn| conn.name == name }
     end
   end

src/granite/table.cr

@@ -10,6 +10,10 @@ end
 
 module Granite::Tables
   module ClassMethods
+    def adapter : Granite::Adapter::Base
+      Granite::Connections.registered_connections.first? || raise "No connections have been registered."
+    end
+
     def primary_name
       {% begin %}
       {% primary_key = @type.instance_vars.find { |ivar| (ann = ivar.annotation(Granite::Column)) && ann[:primary] } %}