Codes fail on round trip marshal/unmarshal

[dackroyd]

Unmarshalling a codes.Code value that was previously marshalled to JSON fails.

Test case reproducing the issue:

func TestRoundTripCodes(t *testing.T) {
	tests := []struct {
		input  codes.Code
	}{
		{codes.Unset},
		{codes.Error},
		{codes.Ok},
	}
	for _, test := range tests {
		c := test.input
		out := new(codes.Code)

		b, err := c.MarshalJSON()
		if err != nil {
			t.Fatalf("Code(%s).MarshalJSON() errored: %v", test.input, err)
		}

		if err := out.UnmarshalJSON(b); err != nil {
			t.Fatalf("Code.UnmarshalJSON(%q) errored: %v", c, err)
		}

		if *out != test.input {
			t.Errorf("failed to round trip %q, output was %v", test.input, out)
		}
	}
}

Fails with (all codes fail, not just Unset):

=== RUN   TestRoundTripCodes
    TestRoundTripCodes: roundtrip_codes_test.go:27: Code.UnmarshalJSON("Unset") errored: invalid code: "\"Unset\""
--- FAIL: TestRoundTripCodes (0.00s)

Unmarshal/marshal should round trip successfully, but the wrapping quotes in the incoming bytes when performing an unmarshal aren't expected by the mapping code, so it fails.