`encode_body`: `:form` and nested values

[wojtekmach]

Before:

Req.post!(req,
  url: "/prices",
  form: [
    product: product.id,
    unit_amount: 100,
    currency: "usd",
    "recurring[interval]": "month"
  ]
)

After:

Req.post!(req,
  url: "/prices",
  form: [
    product: product.id,
    unit_amount: 100,
    currency: "usd",
    recurring: [
      interval: "month"
    ]
  ]
)