Fix issue when writing to empty dictionary.

Author: rogerluan

Sources/KeyPath.swift

@@ -86,7 +86,7 @@ public extension Dictionary where Key == String {
                 // Reached the end of the key path.
                 self[head] = newValue as? Value
             case let (head, remainingKeyPath)?:
-                let value = self[head]
+                let value: Any = self[head] ?? [:]
                 if var nestedDictionary = value as? [Key:Any] {
                     // Key path has a tail we need to traverse
                     nestedDictionary[keyPath: remainingKeyPath] = newValue

Tests/KeyPathTests.swift

@@ -92,4 +92,11 @@ final class KeyPathTests : XCTestCase {
         let rhs = "this.is.path.end"
         XCTAssertEqual(KeyPath(lhs) + KeyPath(rhs), "this.is.path.start.this.is.path.end")
     }
+
+    func test_writingToEmptyDictionary() throws {
+        let data = "this is my data"
+        var dictionary: [String:Any] = [:]
+        dictionary[keyPath: "nested.data"] = data
+        XCTAssertEqual(dictionary[keyPath: "nested.data"] as? String, data)
+    }
 }