Serialisation algorithm causes missing data silently when object key names have dots in them

[rjchow]

> flatten({foo: { bar: "qux" }})
{ 'foo.bar': 'qux' }
> flatten({foo: { bar: "qux" }, bar: "asd" })
{ 'foo.bar': 'qux', bar: 'asd' }
> flatten({foo: { bar: "qux" }, "foo.bar": "asd" })
{ 'foo.bar': 'asd' } // should throw error because it is unable to serialise correctly