URL-encode dates using unix timestamp (vapor#2276)

* Added ability to configure date coding/decoding for UrlEncodedForm

* Added documentation to the `DateFormat` enum

* renamed internetDateTime to iso8601

* Added custom date formatter

* Added comment about `ISO8601DateFormatter` performance

* Added ISO8601DateFormatter.threadSpecific so a new ISO8601DateFormatter isn't created for every encode/decode.

* Use typealias instead of declaring DateFormat 2x

* Added `ThreadSpecificDateFormatter` to ensure thread safety when using a `custom` DateFormat for `URLEncodedFormEncoder` or `URLEncodedFormDecoder`

* Fixed comment

* Changed custom interface to mimic `JSONDecoder.DateDecodingStrategy.custom(_:)` and `JSONEncoder.DateEncodingStrategy.custom(_:)` interfaces

* Removed unused ThreadSpecificDateFormatter

* `ISO8601DateFormatter` seems to be thread safe. This bug was filed: https://bugs.swift.org/browse/SR-7745?page=com.atlassian.jira.plugin.system.issuetabpanels%3Aall-tabpanel
I tried running the sample program with `10000000` iterations in the `vapor/swift:5.2` docker image without any issue.

* Default the date format for URLEncodedFrom coding/decoding to `timeIntervalSinceReferenceDate`

* Default to `timeIntervalSince1970`

* Removed all references to customizing DateFormat and default to `timeIntervalSince1970`

* Removed reference to `import NIO` and fixed spacing

* Fixed spacing

* Implemented using `extension Date: URLQueryFragmentConvertible`

* Spacing

Sources/Vapor/URLEncodedForm/URLQueryFragmentConvertible.swift

@@ -103,3 +103,16 @@ extension Decimal: URLQueryFragmentConvertible {
         return .urlDecoded(self.description)
     }
 }
+
+extension Date: URLQueryFragmentConvertible {
+    init?(urlQueryFragmentValue value: URLQueryFragment) {
+        guard let double = Double(urlQueryFragmentValue: value) else {
+            return nil
+        }
+        self = Date(timeIntervalSince1970: double)
+    }
+
+    var urlQueryFragmentValue: URLQueryFragment {
+        return timeIntervalSince1970.urlQueryFragmentValue
+    }
+}

Tests/VaporTests/URLEncodedFormTests.swift

@@ -114,6 +114,17 @@ final class URLEncodedFormTests: XCTestCase {
         XCTAssert(result.contains("isCool=true"))
     }
 
+    func testDateCoding() throws {
+        let toEncode = DateCoding(date: Date(timeIntervalSince1970: 0))
+        let resultForTimeIntervalSince1970 = try URLEncodedFormEncoder()
+          .encode(toEncode)
+        XCTAssertEqual("date=0.0", resultForTimeIntervalSince1970)
+
+        let decodedTimeIntervalSince1970 = try URLEncodedFormDecoder()
+          .decode(DateCoding.self, from: resultForTimeIntervalSince1970)
+        XCTAssertEqual(decodedTimeIntervalSince1970, toEncode)
+    }
+
     func testEncodedArrayValues() throws {
         let user = User(name: "Tanner", age: 23, pets: ["Zizek", "Foo"], dict: ["a": 1, "b": 2], foos: [.baz], nums: [3.14], isCool: true)
         let result = try URLEncodedFormEncoder(
@@ -469,3 +480,7 @@ private struct Users: Codable, Equatable {
 private enum Foo: String, Codable {
     case foo, bar, baz
 }
+
+struct DateCoding: Codable, Equatable {
+    let date: Date
+}