Allow passing a custom completion queue to ImageDownloader

[eneko]

• I've read, understood, and done my best to follow the CONTRIBUTING guidelines.

This pull request includes (pick all that apply):

• Bugfixes
• New features
• Breaking changes
• Documentation updates
• Unit tests
• Integration tests
• Other

Summary

Allow passing a custom operation queue to ImageDownloader to be used when calling the completion callback.

Implementation

• Add completionQueue parameter to initializer (defaults to nil)
  • If no value is passed, ImageDownloader will behave as before, defaulting to the current operation queue for completion, or main if there is no current queue.
  • When a queue is specified, it will be used instead.

Test Plan

• Run integration tests

[plflanagan]

Looks great!