update ISO8601 utility to remove timezone colon (#76)

Author: tcldr

Sources/Segment/Utilities/iso8601.swift

@@ -7,31 +7,24 @@
 
 import Foundation
 
-var __segment_isoDateFormatter = SegmentISO8601DateFormatter()
-
-class SegmentISO8601DateFormatter: DateFormatter {
-    override init() {
-        super.init()
-        
-        self.dateFormat = "yyyy'-'MM'-'dd'T'HH':'mm':'ss.SSS:'Z'"
-        self.locale = Locale(identifier: "en_US_POSIX")
-        self.timeZone = TimeZone(secondsFromGMT: 0)
-    }
+enum SegmentISO8601DateFormatter {
 
-    required init?(coder: NSCoder) {
-        fatalError("init(coder:) has not been implemented")
-    }
+    static let shared: ISO8601DateFormatter = {
+        let formatter = ISO8601DateFormatter()
+        formatter.formatOptions.update(with: .withFractionalSeconds)
+        return formatter
+    }()
 }
 
 internal extension Date {
     // TODO: support nanoseconds
     func iso8601() -> String {
-        return __segment_isoDateFormatter.string(from: self)
+        return SegmentISO8601DateFormatter.shared.string(from: self)
     }
 }
 
 internal extension String {
     func iso8601() -> Date? {
-        return __segment_isoDateFormatter.date(from: self)
+        return SegmentISO8601DateFormatter.shared.date(from: self)
     }
 }