add more tests to models

spec/support/matchers/model/model_have_error_on_field.rb

@@ -0,0 +1,15 @@
+RSpec::Matchers.define :model_have_error_on_field do |expected|
+  match do |record|
+    if record.errors.empty?
+      record.valid?
+    end
+
+    record.errors.has_key?(expected)
+  end
+
+  failure_message do |record|
+    keys = record.errors.keys
+    
+    "expect record.errors(#{keys}) to include #{expected}"
+  end
+end