Issue Description:

When serializing Double values smaller than 1.0 and bigger than -1.0, the current implementation omits the leading "0". This is not allowed according to json.org and in fact the produced JSON code can't be parsed by the iOS/macOS implementation of NSJSONSerialization!

Example

Incorrect

{
    "foo": .1234
}

Should be

{
   "foo": 0.1234
}