Follow Apple's doc, add NSOperation only after all configuration done.

[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 / reffers to the following issues:

Pull Request Description

Just a little improvement to fix potential issue (Though works well now, but it's like a wrong usage). See Apple's doc this one and this one

Important: You should make all necessary configuration and modifications to an operation object before adding it to a queue, because once added, the operation may be run at any time, which may be too late for a change to have the intended effect.

Important: You should always configure dependencies before running your operations or adding them to an operation queue. Dependencies added afterward may not prevent a given operation object from running.

[codecov-io]

Codecov Report

Merging #2232 into master will increase coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2232      +/-   ##
=========================================
+ Coverage   78.24%   78.5%   +0.25%     
=========================================
  Files          36      36              
  Lines        3733    3735       +2     
  Branches      341     342       +1     
=========================================
+ Hits         2921    2932      +11     
+ Misses        789     780       -9     
  Partials       23      23

Impacted Files	Coverage Δ
SDWebImage/SDWebImageDownloader.m	82.17% <100%> (+0.15%)	⬆️
SDWebImage/SDWebImageDownloaderOperation.m	92.18% <0%> (+2.93%)	⬆️

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