Generate UnmarshalJSON to handle emptyStringInt types

This commit changes the code generator to generate a `UnmarshalJSON` for each
struct, so that if unmarshalled it properly handles UniFis varying integer values
via the `emptyStringInt` type.

Structs not including a field of `int` type will still have the function generated,
but it will effectively do nothing.

Fixes #18

unifi/network_test.go

@@ -51,6 +51,19 @@ func TestNetworkUnmarshalJSON(t *testing.T) {
 			expected: unifi.Network{WANEgressQOS: 0},
 			json:     `{ "wan_egress_qos": "" }`,
 		},
+
+		"int wan_vlan": {
+			expected: unifi.Network{WANVLAN: 1},
+			json:     `{ "wan_vlan": 1 }`,
+		},
+		"string wan_vlan": {
+			expected: unifi.Network{WANVLAN: 1},
+			json:     `{ "wan_vlan": "1" }`,
+		},
+		"empty wan_vlan vlan": {
+			expected: unifi.Network{WANVLAN: 0},
+			json:     `{ "wan_vlan": "" }`,
+		},
 	} {
 		t.Run(n, func(t *testing.T) {
 			var actual unifi.Network