make dupefilter support create from spider

[zoyanhui]

redis dupefilter is based on spider, it may suppport from spider.

[codecov-io]

Current coverage is 96.57% (diff: 100%)

Merging #83 into master will increase coverage by 0.30%

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[rmax]

Thanks for your PR. I will think about this as it will break custom dupefilter classes.

[tyong920]

@zoyanhui I think this PR is NOT needed. If i'm right, you might want to make it possible to set specific settings for scrapy_redis in spider. To do that, you could just set them in custom_settings in your spider.
A simple

custom_settings = {
        ... ...
        'SCHEDULER_DUPEFILTER_KEY' = '... ...',
        ...