class_docstr: WSGI middleware validating security on response.
  
  It validates:
  - responses have Content-Type
  - responses have X-Content-Type-Options: nosniff
  - text/html responses have CSP header. It also validates whether the CSP
    headers pass basic requirement. e.g., default-src should be present, cannot
    use "*" directive, and others. For more complete list, please refer to
    _validate_csp_policies.
  
  Instances of this class are WSGI applications (see PEP 3333).
code: def __call__(self, environ, start_response):
      def start_response_proxy(status, headers, exc_info=None):
          self._validate_headers(headers)
          return start_response(status, headers, exc_info)
  
      return self._application(environ, start_response_proxy)
  
file_path: tensorboard/tensorboard/backend/security_validator.py
imports: import dataclasses
  from typing import Collection
  from werkzeug.datastructures import Headers
  from werkzeug import http
  from tensorboard.util import tb_logging
label_desc: Use image node_id 2 for calling the SecurityValidatorMiddleware obj's underlying member method code with example usage: obj.__call__(environ, start_response) and returns: self, start_response