Add exception for 429 Too Many Requests errors

When rate limits are hit a generic `CreateSend::ClientError` is raised
with no explanatory details, which makes implementing a backoff / retry
mechanism difficult. If the same request is made manually, we may see a
response like:

```
{
  "Code" => 429,
  "Message" => "Subscriber was added too many times too quickly, try again later."
}
```

Which allow us to differentiate between rate limiting errors and other
client errors. This adds a new `CreateSend::TooManyRequests` exception
to be raised when the API responds with such a 429 so we can do so in
code.

`CreateSend::TooManyRequests` inherits from `ClientError`, so any
current code that rescues `ClientError` should be unaffected.

lib/createsend/createsend.rb

@@ -30,6 +30,8 @@ class BadRequest < CreateSendError; end
  class Unauthorized < CreateSendError; end
  # Raised for HTTP response code of 404
  class NotFound < ClientError; end
+ # Raised for HTTP response code of 429
+ class TooManyRequests < ClientError; end
 
  # Raised for HTTP response code of 401, specifically when an OAuth token
  # in invalid (Code: 120, Message: 'Invalid OAuth Token')
@@ -279,6 +281,8 @@ def handle_response(response) # :nodoc:
  end
  when 404
  raise NotFound.new
+ when 429
+ raise TooManyRequests.new
  when 400...500
  raise ClientError.new
  when 500...600

test/createsend_test.rb

@@ -278,11 +278,12 @@ class CreateSendTest < Test::Unit::TestCase
  @template = CreateSend::Template.new @auth, '98y2e98y289dh89h938389'
  end
 
- { ["400", "Bad Request"] => CreateSend::BadRequest,
- ["401", "Unauthorized"] => CreateSend::Unauthorized,
- ["404", "Not Found"] => CreateSend::NotFound,
- ["418", "I'm a teapot"] => CreateSend::ClientError,
- ["500", "Server Error"] => CreateSend::ServerError
+ { ["400", "Bad Request"] => CreateSend::BadRequest,
+ ["401", "Unauthorized"] => CreateSend::Unauthorized,
+ ["404", "Not Found"] => CreateSend::NotFound,
+ ["418", "I'm a teapot"] => CreateSend::ClientError,
+ ["429", "Too many requests"] => CreateSend::TooManyRequests,
+ ["500", "Server Error"] => CreateSend::ServerError
  }.each do |status, exception|
  context "#{status.first}, a get" do
  should "raise a #{exception.name} error" do