Fix completion queue bug in ImageDownloader (#163)

### Summary
In most cases, `ImageDownloader` will be executed from the main thread. However, in scenarios where this might not be the case, we'd want to make sure the completion queue is executed in the `.current` queue (when no custom queue is specified).

### Implementation
Store `.current` queue outside of any closures to ensure the queue is the same the method was called in.

Sources/Conduit/Networking/Images/ImageDownloader.swift

@@ -91,6 +91,7 @@ public final class ImageDownloader: ImageDownloaderType {
     @discardableResult
     public func downloadImage(for request: URLRequest, completion: @escaping CompletionHandler) -> SessionTaskProxyType? {
         var proxy: SessionTaskProxyType?
+        let completionQueue = self.completionQueue ?? .current ?? .main
 
         serialQueue.sync { [weak self] in
             guard let `self` = self else {
@@ -133,10 +134,9 @@ public final class ImageDownloader: ImageDownloaderType {
                 }
 
                 let response = Response(image: image, error: error, urlResponse: response, isFromCache: false)
-                let queue = strongSelf.completionQueue ?? .current ?? .main
 
                 func execute(handler: @escaping CompletionHandler) {
-                    queue.addOperation {
+                    completionQueue.addOperation {
                         handler(response)
                     }
                 }

Tests/ConduitTests/Networking/Images/ImageDownloaderTests.swift

@@ -134,6 +134,28 @@ class ImageDownloaderTests: XCTestCase {
         waitForExpectations(timeout: 5)
     }
 
+    func testCurrentOperationQueue() throws {
+        // GIVEN an operation queue
+        let expectedQueue = OperationQueue()
+
+        // AND a configured Image Downloader instance
+        let imageDownloadedExpectation = expectation(description: "image downloaded")
+        let sut = ImageDownloader(cache: AutoPurgingURLImageCache())
+        let url = try URL(absoluteString: "https://httpbin.org/image/jpeg")
+        let imageRequest = URLRequest(url: url)
+
+        // WHEN downloading an image from our background queue
+        expectedQueue.addOperation {
+            sut.downloadImage(for: imageRequest) { _ in
+                // THEN the completion handler is called in the expected queue
+                XCTAssertEqual(OperationQueue.current, expectedQueue)
+                imageDownloadedExpectation.fulfill()
+            }
+        }
+
+        waitForExpectations(timeout: 5)
+    }
+
     func testCustomOperationQueue() throws {
         // GIVEN a custom operation queue
         let customQueue = OperationQueue()