php_cli_server: ensure single date header is present

Currently the PHP Development Server appends a Date header in the
response, despite already set from user code.

Added a chec

php_cli_server: ensure single date header is present

Currently the PHP Development Server appends a Date header in the
response, despite already set from user code.

Added a check condition before append the header, and a test file.

Closes GH-12363.

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