[4.x] Fix the issue that "There may be no complete callback when download the picture of the local path"

[dreampiggy]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes / refers to the following issues: #2910

Pull Request Description

4.x branch.

[codecov]

Codecov Report

Merging #2916 into 4.x will decrease coverage by 4.74%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##              4.x    #2916      +/-   ##
==========================================
- Coverage   75.15%   70.41%   -4.75%     
==========================================
  Files          37       29       -8     
  Lines        4074     3231     -843     
==========================================
- Hits         3062     2275     -787     
+ Misses       1012      956      -56

Impacted Files	Coverage Δ
SDWebImage/SDWebImageDownloader.m	77.9% <86.66%> (+0.61%)	⬆️
SDWebImage/SDWebImageDownloaderOperation.m	87.08% <0%> (-2.99%)	⬇️
SDWebImage/SDImageCache.m	65.09% <0%> (-0.42%)	⬇️
Tests/Tests/SDWebImageTestDecoder.m
Tests/Tests/SDWebImageDecoderTests.m
Tests/Tests/SDTestCase.m
Tests/Tests/SDCategoriesTests.m
Tests/Tests/SDWebImagePrefetcherTests.m
Tests/Tests/SDWebImageManagerTests.m
Tests/Tests/SDImageCacheTests.m
... and 2 more

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