code: private function checkAuthorize(array $fields)
  	{
  		if ( false === $fields['merchant_id'] )
  		{
  			throw new ApiException(Errors::ERRCODE_MERCHANT_ID_INVALID);
  		}
  		if ( false === $fields['merchant_password'] )
  		{
  			throw new ApiException(Errors::ERRCODE_MERCHANT_PASSWORD_INVALID);
  		}
  		if ( false === $fields['receiver_email'] )
  		{
  			throw new ApiException(Errors::ERRCODE_MERCHANT_EMAIL_INVALID);
  		}
  	}
docs: Check ahthorize
  @internal: called by checkFields function
  
  @param array $fields
  @throw ApiException