Fix and force a missing include of exception on Param Auth Header cla…

…ss. This is available on request classes but but here (#168)

* Fix and force a missing include of exception on Param Auth Header class. This is available on request classes but but here

* Instead of include, use full path

lib/uploadcare/param/authentication_header.rb

@@ -23,8 +23,8 @@ def self.call(options = {})
       end
 
       def self.validate_auth_config
-        raise AuthError, 'Public Key is blank.' if is_blank?(Uploadcare.config.public_key)
-        raise AuthError, 'Secret Key is blank.' if is_blank?(Uploadcare.config.secret_key)
+        raise Uploadcare::Exception::AuthError, 'Public Key is blank.' if is_blank?(Uploadcare.config.public_key)
+        raise Uploadcare::Exception::AuthError, 'Secret Key is blank.' if is_blank?(Uploadcare.config.secret_key)
       end
 
       # rubocop:disable Naming/PredicateName