Returns only proper HTTP headers.

def get_headers(environ):
    """
    Returns only proper HTTP headers.
    """
    for key, value in compat.iteritems(environ):
        key = str(key)
        if key.startswith("HTTP_") and key not in ("HTTP_CONTENT_TYPE", "HTTP_CONTENT_LENGTH"):
            yield key[5:].replace("_", "-").lower(), value
        elif key in ("CONTENT_TYPE", "CONTENT_LENGTH"):
            yield key.replace("_", "-").lower(), value