fix: use default field name when no json tag is provided

[Nazdroth]

Adding default field name when no json tag is provided
for example before :

type UserInfo struct {
    ID string `"groups":"public"`
}

We get as an output:
{ "": "A" }
And now that we use default name if tag is absent we get:
{ "ID": "A" }
which is a similar behaviour as the go default JSON encoder

[coveralls]

Coverage increased (+0.4%) to 82.759% when pulling d36d76a on Nazdroth:noJsonTag into bb4d97c on liip:master.

[mweibel]

Thanks for the fix!