Add encoding the geo point and document reference

CodableFirebase/Encoder.swift

@@ -13,6 +13,7 @@ class _FirebaseEncoder : Encoder {
     struct _Options {
         let dateEncodingStrategy: FirebaseEncoder.DateEncodingStrategy?
         let dataEncodingStrategy: FirebaseEncoder.DataEncodingStrategy?
+        let skipGeoPointAndReference: Bool
         let userInfo: [CodingUserInfoKey : Any]
     }
     
@@ -382,6 +383,11 @@ extension _FirebaseEncoder {
             return try self.box((value as! Data))
         } else if T.self == URL.self || T.self == NSURL.self {
             return self.box((value as! URL).absoluteString)
+        } else if options.skipGeoPointAndReference && (value is GeoPointType || value is DocumentReferenceType) {
+            guard let value = value as? NSObject else {
+                throw DocumentReferenceError.typeIsNotNSObject
+            }
+            return value
         }
         
         // The value should request a container from the _FirebaseEncoder.

CodableFirebase/FirebaseEncoder.swift

@@ -57,6 +57,7 @@ open class FirebaseEncoder {
         let options = _FirebaseEncoder._Options(
             dateEncodingStrategy: dateEncodingStrategy,
             dataEncodingStrategy: dataEncodingStrategy,
+            skipGeoPointAndReference: true,
             userInfo: userInfo
         )
         let encoder = _FirebaseEncoder(options: options)

CodableFirebase/FirestoreDecoder.swift

@@ -58,6 +58,7 @@ extension GeoPointType {
 
 enum DocumentReferenceError: Error {
     case typeIsNotSupported
+    case typeIsNotNSObject
 }
 
 extension DocumentReferenceType {

CodableFirebase/FirestoreEncoder.swift

@@ -26,7 +26,12 @@ open class FirestoreEncoder {
     }
     
     internal func encodeToTopLevelContainer<Value : Encodable>(_ value: Value) throws -> Any {
-        let options = _FirebaseEncoder._Options(dateEncodingStrategy: nil, dataEncodingStrategy: nil, userInfo: userInfo)
+        let options = _FirebaseEncoder._Options(
+            dateEncodingStrategy: nil,
+            dataEncodingStrategy: nil,
+            skipGeoPointAndReference: false,
+            userInfo: userInfo
+        )
         let encoder = _FirebaseEncoder(options: options)
         guard let topLevel = try encoder.box_(value) else {
             throw EncodingError.invalidValue(value,