Review url being optional for SplashRequest

[Gallaecio]

As part of https://github.com/scrapy-plugins/scrapy-splash/pull/269, the url parameter to SplashRequest is no longer optional.

@elacuesta noticed that this is a backward-incompatible change. Moreover, the upcoming Scrapy 2.4 will make this change unnecessary (https://github.com/scrapy/scrapy/pull/4835).

We should consider reverting that part of https://github.com/scrapy-plugins/scrapy-splash/pull/269.

[lopuhin]

sounds good to me, thanks for the heads up @Gallaecio

[Rafiot]

Reverting https://github.com/scrapy-plugins/scrapy-splash/pull/269/commits/4154782b16c6248a5a9b8fe3ed0ebb1cbc600a60 will do just that, so just go for it.