intersections don't play well with exact

[gcanti]

🐛 Bug report

Reproducible example

const C = t.intersection([t.exact(t.type({})), t.exact(t.partial({ a: t.number }))])
assert.deepStrictEqual(C.decode({ a: 1 }), right({ a: 1 }))
assert.deepStrictEqual(C.decode({ a: 1, b: 1 }), right({ a: 1 }))

Additional context

RE: #390

Your environment

Software	Version(s)
io-ts	2.0.1
fp-ts	2.0.0
TypeScript	3.7.2