Also set the webURL to nil when resource is nil

[pNre]

Hi,
while using Kingfisher in a collection view I noticed that in some cases the image views weren't updated as expected.

This is what I'm doing:

• Dequeue a reusable cell containing an image view
• Set the image to a certain image-url using KF
• At some point the cell gets reused and the download KF download canceled
• Set the image to nil with a placeholder

What I expect at this point is for the image view to display the placeholder, what happens is that it still displays the image at image-url.

I looked a bit into the setImage method in ImageView+Kingfisher and noticed that the cause of this issue might be the fact that webURL is not updated when setImage is called with a nil resource thus letting the completion block of the download task update the image even if the operation was cancelled and the setImage called again.

Also, the same this is probably happening for UIButton and NSButton extensions but I wanted to open this PR before editing any other method.

[codecov-io]

Codecov Report

Merging #598 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
+ Coverage   74.71%   74.72%   +0.01%     
==========================================
  Files          21       21              
  Lines        2246     2247       +1     
==========================================
+ Hits         1678     1679       +1     
  Misses        568      568

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[onevcat]

@pNre Good catch. I believe you are right on this.

Could you please also send p-r for button types too? Thank you!