Adds a basic spec for #create_bill

It looks like it doesn't work that way:

```ruby
Failure/Error: preauth.create_bill(mocked_data)
       #<GoCardless::Client:0x007fe124141600> received :api_post with unexpected arguments
         expected: ("/bills", {:bill=>{:pre_authorization_id=>"009988", :amount=>"15.00", :name=>"150 credits", :charge_customer_at=>#<DateTime: 2013-08-27T00:00:00+00:00 ((2456532j,0s,0n),+0s,2299161j)>}})
              got: ("/bills/", {:bill=>{:pre_authorization_id=>"009988", :amount=>"15.00", :name=>"150 credits", :charge_customer_at=>#<DateTime: 2013-08-27T00:00:00+00:00 ((2456532j,0s,0n),+0s,2299161j)>}})
```