Adds support for the RFC2822 date format (marksands#9)

Sources/BetterCodable/RFC2822Strategy.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+/// Decodes `String` values as an RFC 2822 `Date`.
+///
+/// `@RFC2822Date` decodes RFC 2822 date strings into `Date`s. Encoding the `Date` will encode the value back into the original string value.
+///
+/// For example, decoding json data with a `String` representation  of `"Tue, 24 Dec 2019 16:39:57 -0000"` produces a valid `Date` representing 39 minutes and 57 seconds
+/// after the 16th hour of December 24th, 2019 with an offset of -00:00 from UTC (Pacific Standard Time).
+public struct RFC2822Strategy: DateValueCodableStrategy {
+    private static let dateFormatter: DateFormatter = {
+        let dateFormatter = DateFormatter()
+        dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
+        dateFormatter.locale = Locale(identifier: "en_US_POSIX")
+        dateFormatter.dateFormat = "EEE, d MMM y HH:mm:ss zzz"
+        return dateFormatter
+    }()
+
+    public static func decode(_ value: String) throws -> Date {
+        if let date = RFC2822Strategy.dateFormatter.date(from: value) {
+            return date
+        } else {
+            throw DecodingError.dataCorrupted(.init(codingPath: [], debugDescription: "Invalid Date Format!"))
+        }
+    }
+
+    public static func encode(_ date: Date) -> String {
+        return RFC2822Strategy.dateFormatter.string(from: date)
+    }
+}

Tests/BetterCodableTests/DateValueTests.swift

@@ -21,6 +21,16 @@ class CustomDateCodableValueTests: XCTestCase {
         let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
         XCTAssertEqual(fixture.rfc3339Date, Date(timeIntervalSince1970: 851042397))
     }
+
+    func testDecodingAndEncodingRFC2822DateString() throws {
+         struct Fixture: Codable {
+            @DateValue<RFC2822Strategy> var rfc2822Date: Date
+        }
+        let jsonData = #"{"rfc2822Date": "Fri, 27 Dec 2019 22:43:52 -0000"}"#.data(using: .utf8)!
+
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+        XCTAssertEqual(fixture.rfc2822Date, Date(timeIntervalSince1970: 1577486632))
+    }
 
     func testDecodingAndEncodingUTCTimestamp() throws {
         struct Fixture: Codable {