Fixes TypeError caused by attempts to Base64 encode nil @api_key values

[alexfulsome]

When a Client is initialized with a nil api_key value (a Dotenv loading issue in my case), requests fail with an opaque TypeError when #set_headers tries to base64 encode it:

[2] Pry(main)> subscriptions.first
TypeError: no implicit conversion of nil into String
from /usr/local/var/rbenv/versions/2.5.7/lib/ruby/2.5.0/base64.rb:39:in `pack'

This explicitly casts @api_key to a string, which makes a blank value raise a Recurly::Errors::UnauthorizedError instead.

[douglasmiller]

Good catch, @alexfulsome.

Let's try to save a trip to the API and add something like raise ArgumentError, "'api_key' must be set to a non-nil value" if api_key.nil? to the initializer prior to calling set_api_key

[douglasmiller]

👍