[DOC] proto void header_remove([string header_name])
Removes an HTTP header previously set using header()
The header_name parameter is optionnal, all headers are
removed if it is not set
[SAPIs] The header_handler callback in sapi_module_struct has
been changed, it now take a new argument.
When it is set to SAPI_HEADER_DELETE, sapi_header->header is
the name of an header, header_handler has to delete it.
When it is set to SAPI_HEADER_DELETE_ALL, header_handler has
to delete all headers.
When sapi_header_op_enum is SAPI_HEADER_ADD or _REPLACE,
sapi_header->header is in the form "Name: value", header_handler
has to add or replace the given header.
In all cases, header_handler must not free sapi_header or
sapi_header->header. SAPI_HEADER_ADD must be returned if the
header has been added or replaced, or 0 in other cases.
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