Add Swift test and fixes

Author: ncooke3

FirebaseFunctions/Sources/HTTPSCallable.swift

@@ -79,7 +79,7 @@ open class HTTPSCallable: NSObject {
                                                    completion: @escaping (HTTPSCallableResult?,
                                                                           Error?) -> Void) {
     if #available(iOS 13, macCatalyst 13, macOS 10.15, tvOS 13, watchOS 7, *) {
-      Task {
+      Task { @MainActor in
         do {
           let result = try await call(data)
           completion(result, nil)

FirebaseFunctions/Tests/Integration/IntegrationTests.swift

@@ -867,6 +867,38 @@ class IntegrationTests: XCTestCase {
       XCTAssertEqual(response, expected)
     }
   }
+
+  func testFunctionsReturnsOnMainThread() {
+    let expectation = expectation(description: #function)
+    functions.httpsCallable(
+      "scalarTest",
+      requestAs: Int16.self,
+      responseAs: Int.self
+    ).call(17) { result in
+      guard case .success = result else {
+        return XCTFail("Unexpected failure.")
+      }
+      XCTAssert(Thread.isMainThread)
+      expectation.fulfill()
+    }
+    waitForExpectations(timeout: 5)
+  }
+
+  func testFunctionsThrowsOnMainThread() {
+    let expectation = expectation(description: #function)
+    functions.httpsCallable(
+      "httpErrorTest",
+      requestAs: [Int].self,
+      responseAs: Int.self
+    ).call([]) { result in
+      guard case .failure = result else {
+        return XCTFail("Unexpected failure.")
+      }
+      XCTAssert(Thread.isMainThread)
+      expectation.fulfill()
+    }
+    waitForExpectations(timeout: 5)
+  }
 }
 
 // MARK: - Streaming

FirebaseFunctions/Tests/Unit/FunctionsTests.swift

@@ -290,7 +290,7 @@ class FunctionsTests: XCTestCase {
         }
 
         XCTAssertEqual(error as NSError, networkError)
-
+        XCTAssert(Thread.isMainThread)
         completionExpectation.fulfill()
       }