Fix decoding of GateStatusDL

[ccascone]

And also add new tests to cover all possible encoding/decoding combinations.

Test failures without the fix:

➜  go-pfcp git:(fix-gate-status-decode) ✗ go test ie/ie_uint8_test.go 
--- FAIL: TestUint8IEs (0.01s)
    --- FAIL: TestUint8IEs/GateStatus/ClosedOpen/GateStatusDL (0.00s)
        ie_uint8_test.go:1245:   uint8(
            -   4,
            +   0,
              )

    --- FAIL: TestUint8IEs/GateStatus/ClosedClosed/GateStatusDL (0.00s)
        ie_uint8_test.go:1245:   uint8(
            -   5,
            +   1,
              )

[wmnsk]

tagged v0.0.13.