def validate_credentials!
      credentials.each do |credential, value|
        next if value.nil?

        unless value.is_a?(String) || value.is_a?(::Symbol)
          raise(Error::ConfigurationError, "Invalid #{credential} specified: #{value} must be a string or symbol.")
        end
      end
    end
def validate_credentials!
      credentials.each do |credential, value|
        next if value.nil?

        unless value.is_a?(String) || value.is_a?(::Symbol)
          raise(Error::ConfigurationError, "Invalid #{credential} specified: #{value} must be a string or symbol.")
        end
      end
    end
Ensures that all credentials set during configuration are of a
 valid type. Valid types are String and Symbol.

 @raise [Tradier::Error::ConfigurationError] Error is raised when
   supplied Tradier credentials are not a String or Symbol.