Fix decoding single config values

[kusamakura]

Decoding configuration keys one by one, in contrast to decoding the whole thing into a case class, always results in an error:

import com.typesafe.config.ConfigFactory
import io.circe.config.parser

val config = ConfigFactory.parseString("a = 1")
// Returns: Left(io.circe.ParsingFailure: String: 1: a has type NUMBER rather than OBJECT)
// Expected: Right(1)
parser.decodePath[Int](config, "a")

The problem stems from the Config#root call in the toJson method in the parser, where a pathed Config may hold a string, and int, etc., and thus is not a ConfigObject. The fix is to call Config#getValue(path) when the path is present, and Config#root otherwise.

[travisbrown]

This looks reasonable to me but I'll defer to @jonas.