title: CoinBase API - et_exchange_rates and get_currencies failure.
body: <p>Hi when writing a simple python script to test Coinbase Features as per documentation I am getting the following results: </p>
  
  <p><a href="https://i.stack.imgur.com/1pEqx.png" rel="nofollow noreferrer">get_exchange_rates(currency='GBP')</a></p>
  
  <p>I encounter a similar problem when I try to get the price of a given currency that is not BTC-USD:</p>
  
  <pre><code>price = client.get_buy_price(currency_pair = 'BTC-GBP')
  print(price)
  </code></pre>
  
  <p>RESULT: </p>
  
  <blockquote>
    <p>{   "amount": "13788.39",   "base": "BTC",   "currency": "USD" }</p>
  </blockquote>
  
  <p>Is this a problem with the API, or am I doing this incorrectly? </p>
  
  <p>**Additional info:**Using API keys with all functionality allowed.</p>