deprecate read/write object attributes

[sherrardb]

long-term plan note:

there is a mix of objects whose attributes are all 'ro' and those with some subset of attributes that are 'rw'. i think that the argument for deprecation has merit solely on the basis of interface consistency. but my bigger concern is the likelihood of user confusion when allowing updates to the attributes of a retrieved object.

a simple example is

my $customer = $stripe->get_customer( customer_id=> $customer_id);
$customer->email( $new_email_address );
...
# code that makes use of $customer->email()

where the user has not re-posted the customer object.

and the fact that this doesn't behave as expected is just as bad:

~$ perl -Mstrict -Mwarnings -mNet::Stripe -e 'my $stripe = Net::Stripe->new(api_key => $ENV{STRIPE_API_KEY});
my $customer = $stripe->post_customer(
  email=> "john.doe\@example.com"
);
print $customer->email()."\n";
$stripe->post_customer(
  customer=> $customer,
  email=> "dr.john.doe\@example.com"
);
print $customer->email()."\n";'
john.doe@example.com
john.doe@example.com

it is also worth noting that we are significantly lacking in unit tests that verify the expected behavior when updating object attributes and re-posting.