Coding dictionaries (#125)

* Add test for empty dictionary and some comments
* removing invalid pre-condition - this is a valid scenarion when encoding an empty dictionary

---------

Co-authored-by: jberling <johan@dynamictype.se>

Sources/Automerge/Codable/Encoding/AutomergeEncoderImpl.swift

@@ -53,7 +53,6 @@ final class AutomergeEncoderImpl {
         if codingPath.map({ AnyCodingKey($0) }).starts(with: prefix) {
             switch containerType {
             case .Key:
-                precondition(!mapKeysWritten.isEmpty)
                 // Remove keys that exist in this objectId that weren't
                 // written during encode. (clean up 'dead' keys from maps)
                 let extraAutomergeKeys = document.keys(obj: objectIdForContainer)

Tests/AutomergeTests/CodableTests/AutomergeDictionaryEncodeDecodeTests.swift

@@ -19,20 +19,47 @@ final class AutomergeDictionaryEncodeDecodeTests: XCTestCase {
         }
 
         let encoder = AutomergeEncoder(doc: doc)
-        let decoder = AutomergeDecoder(doc: doc)
 
         var wrapper = Wrapper()
         wrapper.exampleDictionary[1] = "one"
         wrapper.exampleDictionary[2] = "two"
 
-        XCTExpectFailure("Automerge encoder can't encode keys other than strings.")
-        try encoder.encode(wrapper)
+        XCTAssertThrowsError(try encoder.encode(wrapper))
+        // Automerge throws a DocError because it only supports
+        // Strings as keys for dictionaries.
+        // DocError(inner: AutomergeUniffi.DocError.WrongObjectType(message: "WrongObjectType"))
+    }
+
+    func testEmptyDictionaryEncode() throws {
+        // example of Dictionary with keys other than a string
+
+        struct Wrapper: Codable, Equatable {
+            var exampleDictionary: [String: Int] = [:]
+        }
+
+        let encoder = AutomergeEncoder(doc: doc)
+        let decoder = AutomergeDecoder(doc: doc)
+        let wrapper = Wrapper()
 
+        try encoder.encode(wrapper)
         let replica = try decoder.decode(Wrapper.self)
         XCTAssertEqual(replica, wrapper)
     }
 
-    func testEmptyDictionaryEncode() throws {
+    func testEmptyDictionaryEncodeAtRoot() throws {
+        let exampleDictionary: [String: Int] = [:]
+
+        let encoder = AutomergeEncoder(doc: doc)
+        let decoder = AutomergeDecoder(doc: doc)
+
+        // this fails when you encode an empty dictionary
+        try encoder.encode(exampleDictionary)
+
+        let replica = try decoder.decode([String: Int].self)
+        XCTAssertEqual(replica, exampleDictionary)
+    }
+
+    func testWorkingDictionaryEncode() throws {
         var exampleDictionary: [String: Int] = [:]
 
         exampleDictionary["one"] = 1