Add default HTTP User-Agent for specific system

[zhongwuzw]

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This merge request fixes / reffers to the following issues: ...

Pull Request Description

Let's tell the server what we are. Server maybe forbidden us if he think it's not the valid request.

Don't needs to reinventing the wheel, borrow snippet from AFNetWorking.

Fixed #2408 .

[codecov-io]

Codecov Report

Merging #2409 into master will decrease coverage by 0.08%.
The diff coverage is 78.57%.

@@            Coverage Diff            @@
##           master   #2409      +/-   ##
=========================================
- Coverage   75.09%     75%   -0.09%     
=========================================
  Files          36      36              
  Lines        3895    3905      +10     
=========================================
+ Hits         2925    2929       +4     
- Misses        970     976       +6

Impacted Files	Coverage Δ
SDWebImage/SDWebImageDownloader.m	78.09% <78.57%> (-0.45%)	⬇️
SDWebImage/SDWebImageDownloaderOperation.m	85.43% <0%> (-1.95%)	⬇️
SDWebImage/SDImageCache.m	63.84% <0%> (-0.42%)	⬇️
Tests/Tests/SDWebImageDownloaderTests.m	83.75% <0%> (+0.9%)	⬆️
SDWebImage/SDWebImageManager.m	75.22% <0%> (+1.35%)	⬆️

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[dreampiggy]

User-Agent it's just a HTTP header.

For 4.x, use headersFilter instead.

For 5.x, use requestModifier instead.

We should not bind complicated business code to a common library. If the user desired to use something, we just provide a abstract protocol or way to let them provide the information.

[zhongwuzw]

@dreampiggy It has no any business code, it just add the device and system version. just like all common user agent. you can see UIWebView/WKWebView or Safari.

Like below, you can see how Apple uses the user-agent to identify the device.

[bpoplauschi]

@zhongwuzw I agree we need to set the User-Agent. I would also consider the comment from @dreampiggy and actually make use of our existing features to do so.
Just that we need this User-Agent configured properly for all our users. We can't expect them to do it. We only offer ways to alter this behavior, if they need that.

[zhongwuzw]

@bpoplauschi the propose @dreampiggy suggested is used for custom for each request, so any opinion about this PR?

[bpoplauschi]

I've looked at this PR again. Guys, I think an API like requestModifier is too advanced for the users that simply use our UIImageView (or similar) categories. I see AFNetworking also sets the User-Agent in the same way, so I say we do the same.
This is not a case where some users want some custom behaviour. From my point of view, this is the User-Agent set for each request of all our users, so we need to set it properly.
The only feature I say we could add (but it's just nice to have) is another field in the DownloaderConfig that allows passing in the User-Agent.

[zhongwuzw]

@bpoplauschi Emm 🤔 , User-Agent is only a field of Headers, user can override these values by (void)setValue:(nullable NSString *)value forHTTPHeaderField:(nullable NSString *)field of SDWebImageDownloader or requestModifier, I think it's maybe enough. Would you think we must add the Boolean in DownloaderConfig?

[dreampiggy]

This seems does not handle tvOS, so it's better to use SD_UIKIT instead.

[zhongwuzw]

Nice catch.

[dreampiggy]

@zhongwuzw Don't need to add one extra config in SDWebImageDownloadConfig or somewhere. It shuold be one of built-in HTTP headers like Accept-Content-Type. User can use current APIs like setValue:forHTTPHeader: or headersFilter to change or override it. It's not suitable to add any property just for User-Agent this specify headers.

[bpoplauschi]

You are right guys, I forgot about setValue:forHTTPHeader:. That is sufficient for the users to customize. So we just need to calculate the User-Agent for the default case, which is most of the users (that will never deal with that). Basically this PR + the minor change request by @dreampiggy

[bpoplauschi]

I see no reason to keep this un-merged. Good job @zhongwuzw