Set HTTPMaximumConnectionsPerHost to PINRemoteImageHTTPMaximumConnectionsPerHost only if user don't provide sessionConfiguration

[zhongwuzw]

If user provide its own sessionConfiguration, we need to respect their configuration, so here we set HTTPMaximumConnectionsPerHost to PINRemoteImageHTTPMaximumConnectionsPerHost,but it may not always user expected, and HTTPMaximumConnectionsPerHost is for any url which belongs to the same host, so it's different from MaxConcurrentOperations.

The original commit is from #342, but I think HTTPMaximumConnectionsPerHost little than MaxConcurrentOperations not means it would always timeout, because they are different concepts. :) And in weak network quality, I think we need to constrain the tcp connections, not set maximum connections to a unlimited number.

[bolsinga]

Please add a test.

[garrettmoon]

I think you're right that this is a good change. I'm wondering if we should update the documentation in the header to mention that users don't want to forget to set it if they provide they're own sessionConfiguration?

[zhongwuzw]

@garrettmoon Yep, I agree, tell users to set HTTPMaximumConnectionsPerHost if they provide their own sessionConfiguration.

[bolsinga]

Thanks for the test!

[garrettmoon]

@zhongwuzw do you mind adding an entry to CHANGELOG.md for this one?

[zhongwuzw]

Changelog added.

[ghost]

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[ghost]

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[garrettmoon]

Mind rebasing against master? I think I fixed a flakey test.

[ghost]

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