Extend date decoding strategy for possible fractional seconds (#35)

* Add support for fractional seconds in dates

If the WriteFreely instance returns dates with fractional seconds (e.g., 2022-09-28T11:55:39.886581893Z), the date-decoding strategy fails and an error is thrown.

* Switch formatOptions depending on fractional seconds

* Bump minimum OS versions

Package.swift

@@ -6,8 +6,8 @@ import PackageDescription
 let package = Package(
     name: "WriteFreely",
     platforms: [
-        .macOS(.v10_12),
-        .iOS(.v10)
+        .macOS(.v10_13),
+        .iOS(.v11)
     ],
     products: [
         // Products define the executables and libraries produced by a package, and make them visible to other packages.

Sources/WriteFreely/Extensions/JSONDecoder+Extension.swift

@@ -0,0 +1,33 @@
+// Credit: https://github.com/vapor/vapor/issues/2481#issuecomment-702013846
+
+import Foundation
+
+extension JSONDecoder.DateDecodingStrategy {
+
+    /// The strategy that formats dates according to the ISO 8601 standard.
+    /// - Note: This includes the fractional seconds, unlike the standard `.iso8601`, which fails to decode those.
+    static var iso8601WithPossibleFractionalSeconds: JSONDecoder.DateDecodingStrategy {
+        JSONDecoder.DateDecodingStrategy.custom { (decoder) in
+            let singleValue = try decoder.singleValueContainer()
+            let dateString  = try singleValue.decode(String.self)
+
+            let formatter = ISO8601DateFormatter()
+
+            /// Use the `.withFractionalSeconds` option only if we have fractional seconds in the date string.
+            if dateString.contains(".") {
+                formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+            } else {
+                formatter.formatOptions = [.withInternetDateTime]
+            }
+
+            guard let date = formatter.date(from: dateString) else {
+                throw DecodingError.dataCorruptedError(
+                    in: singleValue,
+                    debugDescription: "Failed to decode string to ISO 8601 date."
+                )
+            }
+            return date
+        }
+    }
+
+}

Sources/WriteFreely/WFClient.swift

@@ -31,7 +31,7 @@ public class WFClient {
     ///   - session: The URL session to use for connections; defaults to `URLSession.shared`.
     public init(for instanceURL: URL, with session: URLSessionProtocol = URLSession.shared) {
         decoder = JSONDecoder()
-        decoder.dateDecodingStrategy = .iso8601
+        decoder.dateDecodingStrategy = .iso8601WithPossibleFractionalSeconds
 
         self.session = session