Fix test availability.

Tests/ParameterEncoderTests.swift

@@ -105,9 +105,9 @@ final class JSONParameterEncoderTests: BaseTestCase {
     }
 }
 
-@available(macOS 10.13, iOS 11.0, tvOS 11.0, watchOS 4.0, *)
 final class SortedKeysJSONParameterEncoderTests: BaseTestCase {
     func testTestJSONEncoderSortedKeysHasSortedKeys() throws {
+        guard #available(macOS 10.13, iOS 11.0, tvOS 11.0, watchOS 4.0, *) else { return }
         // Given
         let encoder = JSONParameterEncoder.sortedKeys
         let request = URLRequest.makeHTTPBinRequest()