Fix timezone generator for DateTimeOffset (#458)

Author: eugene-g

src/FsCheck/Arbitrary.fs

@@ -776,9 +776,9 @@ module Arb =
         /// A DateTimeOffset is shrunk first by shrinking its offset, then by removing its second, minute and hour components.
         static member DateTimeOffset() =
             let genTimeZone = gen {
-                                let! hours = Gen.choose(-14, 14)
-                                let! minutes = 
-                                    if abs hours = 14 then 
+                                let! hours = Gen.choose(-12, 14)
+                                let! minutes =
+                                    if hours = -12 || hours = 14 then
                                         Gen.constant 0
                                     else 
                                         Gen.choose(0, 59)