Returns only proper HTTP headers.

def _get_headers(environ):
    # type: (Dict[str, str]) -> Iterator[Tuple[str, str]]
    """
    Returns only proper HTTP headers.

    """
    for key, value in environ.items():
        key = str(key)
        if key.startswith("HTTP_") and key not in (
            "HTTP_CONTENT_TYPE",
            "HTTP_CONTENT_LENGTH",
        ):
            yield key[5:].replace("_", "-").title(), value
        elif key in ("CONTENT_TYPE", "CONTENT_LENGTH"):
            yield key.replace("_", "-").title(), value