Get rid of global iso8601 formatter (#274)

Author: adam-fowler

Sources/HummingbirdFoundation/Codable/URLEncodedForm/URLEncodedForm.swift

@@ -45,12 +45,4 @@ internal enum URLEncodedForm {
 
     /// ASCII characters that will not be percent encoded in URL encoded form data
     static let unreservedCharacters = CharacterSet(charactersIn: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.~")
-
-    @available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
-    /// ISO8601 data formatter used throughout URL encoded form code
-    static var iso8601Formatter: ISO8601DateFormatter = {
-        let formatter = ISO8601DateFormatter()
-        formatter.formatOptions = .withInternetDateTime
-        return formatter
-    }()
 }

Sources/HummingbirdFoundation/Codable/URLEncodedForm/URLEncodedFormDecoder.swift

@@ -590,8 +590,10 @@ extension _URLEncodedFormDecoder {
             return Date(timeIntervalSince1970: seconds)
         case .iso8601:
             if #available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *) {
+                let iso8601Formatter = ISO8601DateFormatter()
+                iso8601Formatter.formatOptions = .withInternetDateTime
                 let dateString = try unbox(node, as: String.self)
-                guard let date = URLEncodedForm.iso8601Formatter.date(from: dateString) else {
+                guard let date = iso8601Formatter.date(from: dateString) else {
                     throw DecodingError.dataCorrupted(.init(codingPath: self.codingPath, debugDescription: "Invalid date format"))
                 }
                 return date

Sources/HummingbirdFoundation/Codable/URLEncodedForm/URLEncodedFormEncoder.swift

@@ -324,7 +324,9 @@ extension _URLEncodedFormEncoder {
             try self.encode(Double(date.timeIntervalSince1970).description)
         case .iso8601:
             if #available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *) {
-                try encode(URLEncodedForm.iso8601Formatter.string(from: date))
+                let iso8601Formatter = ISO8601DateFormatter()
+                iso8601Formatter.formatOptions = .withInternetDateTime
+                try encode(iso8601Formatter.string(from: date))
             } else {
                 preconditionFailure("ISO8601DateFormatter is unavailable on this platform")
             }