Make new Swift rfc3339DateFormatter a static let instead of func

It didn't work in Objective-C, but in Swift it's possible to do it and
it makes more sense to have it implemented as `static let` so it's
initialized only once.

Author: mokagio

WordPressKit/NSDate+RFC3339.swift

@@ -2,11 +2,8 @@ import Foundation
 
 extension NSDate {
 
-    // TODO: Make `static let` after conversion since it's not used outside WordPressKit
     @objc
-    static func rfc3339DateFormatter() -> DateFormatter {
-        DateFormatter.rfc3339Formatter
-    }
+    static let rfc3339DateFormatter = DateFormatter.rfc3339Formatter
 
     /// Parses a date string
     ///
@@ -20,12 +17,12 @@ extension NSDate {
     // Needs to be `public` because of the usages in the Objective-C code.
     @objc(dateWithWordPressComJSONString:)
     public static func with(wordPressComJSONString jsonString: String) -> Date? {
-        self.rfc3339DateFormatter().date(from: jsonString)
+        self.rfc3339DateFormatter.date(from: jsonString)
     }
 
     @objc(WordPressComJSONString)
     public func wordPressComJSONString() -> String {
-        NSDate.rfc3339DateFormatter().string(from: self as Date)
+        NSDate.rfc3339DateFormatter.string(from: self as Date)
     }
 }
 

WordPressKit/PostServiceRemoteREST+Extended.swift

@@ -40,7 +40,7 @@ private func decodePost(from object: AnyObject) async throws -> RemotePost {
 
 private func makeParameters<T: Encodable>(from value: T) throws -> [String: AnyObject] {
     let encoder = JSONEncoder()
-    encoder.dateEncodingStrategy = .formatted(NSDate.rfc3339DateFormatter())
+    encoder.dateEncodingStrategy = .formatted(NSDate.rfc3339DateFormatter)
     let data = try encoder.encode(value)
     let object = try JSONSerialization.jsonObject(with: data)
     guard let dictionary = object as? [String: AnyObject] else {

WordPressKitTests/NSDate+RFC3339Tests.swift

@@ -10,7 +10,7 @@ import XCTest
 class NSDateRFC3339Tests: XCTestCase {
 
     func testDateFormatterConfiguration() throws {
-        let rfc3339Formatter = try XCTUnwrap(NSDate.rfc3339DateFormatter())
+        let rfc3339Formatter = try XCTUnwrap(NSDate.rfc3339DateFormatter)
 
         XCTAssertEqual(rfc3339Formatter.timeZone, TimeZone(secondsFromGMT: 0))
         XCTAssertEqual(rfc3339Formatter.locale, Locale(identifier: "en_US_POSIX"))