Incorrect encoding of optional post parameters

[gabro]

The change introduced in #70 does not take into account optional parameters.

Suppose you have a POST route that takes a single optional parameter, then metarpheus-io-ts produces this output, that does not compile:

// ...
data: {
  foo: m.Foo.encode(foo)
}
// ...

the correct encoding would be

// ...
data: {
  foo: t.union([m.Foo, t.undefined]).encode(foo)
}
// ...