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.

[coveralls]

coverage: 89.212% (+0.05%) from 89.167%
when pulling 5579d0b on alecsammon:fix_embedded
into e127e90 on liip:master.

[coveralls]

coverage: 89.212% (+0.05%) from 89.167%
when pulling 80f0f5c on alecsammon:fix_embedded
into dc3eac4 on liip:master.

[alecsammon]

❤️ - thank you

[mweibel]

sorry, got off my radar and just came across this because of another PR coming in :)

[alecsammon]

no problem at all - we've been using our own fork for the moment - but can now switch back to use this.