Add Arbitrary conformance for UnknownJSONAPIError. Fix weird encoding exception that I cannot quite figure out or explain at the moment.

Sources/JSONAPIArbitrary/Error+Arbitrary.swift

@@ -0,0 +1,15 @@
+//
+//  Error+Arbitrary.swift
+//  JSONAPIArbitrary
+//
+//  Created by Mathew Polzin on 1/21/19.
+//
+
+import SwiftCheck
+import JSONAPI
+
+extension UnknownJSONAPIError: Arbitrary {
+	public static var arbitrary: Gen<UnknownJSONAPIError> {
+		return Gen.pure(.unknownError)
+	}
+}