Fix handling decimal value

Author: Oleksii Dykan

CodableFirebase/Encoder.swift

@@ -383,6 +383,8 @@ 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 T.self == Decimal.self || T.self == NSDecimalNumber.self {
+            return (value as! NSDecimalNumber)
         } else if options.skipFirestoreTypes && (value is FirestoreEncodable) {
             guard let value = value as? NSObject else {
                 throw DocumentReferenceError.typeIsNotNSObject

CodableFirebaseTests/TestCodableFirestore.swift

@@ -109,6 +109,11 @@ class TestCodableFirestore: XCTestCase {
         _testRoundTrip(of: TopLevelWrapper(date), expected: ["value": date])
     }
 
+    func testDecimalValue() {
+        let value = Decimal(2)
+        _testRoundTrip(of: TopLevelWrapper(value), expected: ["value": value])
+    }
+    
     // MARK: - GeoPoint & Document Reference
     func testEncodingGeoPoint() {
         let point = GeoPoint(latitude: 2, longitude: 2)