Save custom attribute like wait_time in SeleniumRequest into Request.meta

[laggardkernel]

The canonical way to share info between Request and download middleware is Request.meta.
Custom attributes Request.custom_attr should be avoided, cause it may be dropped
after possible serialization and de-serialization in Scheduler.

E.g. scrapy-redis converts Request into dict with scrapy.utils.reqpar.request_to_dict(). Custom attribute on Request will be lost.

# scrapy.utils.reqpar
def request_to_dict(request, spider=None):
    """Convert Request object to a dict.

    If a spider is given, it will try to find out the name of the spider method
    used in the callback and store that as the callback.
    """
    cb = request.callback
    if callable(cb):
        cb = _find_method(spider, cb)
    eb = request.errback
    if callable(eb):
        eb = _find_method(spider, eb)
    d = {
        'url': to_unicode(request.url),  # urls should be safe (safe_string_url)
        'callback': cb,
        'errback': eb,
        'method': request.method,
        'headers': dict(request.headers),
        'body': request.body,
        'cookies': request.cookies,
        'meta': request.meta,
        '_encoding': request._encoding,
        'priority': request.priority,
        'dont_filter': request.dont_filter,
        'flags': request.flags,
        'cb_kwargs': request.cb_kwargs,
    }
    if type(request) is not Request:
        d['_class'] = request.__module__ + '.' + request.__class__.__name__
    return d