Write tests for the gem

[hammady]

Use rspec to write tests for methods in the two main classes:

1. AmazonSesMailer::Base:

• deliveries
• register_interceptor
• default
• initialize
• method_missing
• mail
• process_merge_vars
• convert_instance_variables_to_merge_vars
• transform_hash
• transform_array
• transform_value
• instance_variable_names

2. AmazonSesMailer::Message:

• ses_client
• initialize
• deliver
• build_message
• ensure_array
• build_list_management_options
• delivering?