Fix embedded struct marshalling

[alecsammon]

Fixes struct marshalling for embedded structs when a json tag is specified.

Currently the following case works, and produces the same result as json.Marshal

type EmbeddedParent struct {
	*EmbeddedChild
}

However this case does not work as expected

type EmbeddedParent struct {
	*EmbeddedChild `json:"embedded"`
}

In the second case the outputted json is flattened, instead of nested as expected.