Try to convert string value in Double/Float decoders

[knutwalker]

From the Gitter conversation.

The Decoders for Int, Long and other integral types try to convert a string into their number type.

import io.circe._, parse._, cats.data.Xor
scala> val Xor.Right(i) = decode[Int]("\"42\"")
i: Int = 42

The decoder for Double and Float don't try this conversion because of this:

scala> "Infinity".toDouble
res1: Double = Infinity

However, we could use JsonNumber.fromString and _.toDouble to try to safely convert a String into a Double, for example:

scala> val doubleWithFromString = Decoder.instance(c => c.as[String].flatMap(s => Xor.fromOption(JsonNumber.fromString(s).map(_.toDouble), DecodingFailure("Double", c.history))))
doubleWithFromString: io.circe.Decoder[Double] = io.circe.Decoder$$anon$8@47016c8b

scala> val Xor.Right(d) = decode("\"13.37\"")(doubleWithFromString)
d: Double = 13.37

scala> val Xor.Left(e) = decode("\"Infinity\"")(doubleWithFromString)
e: io.circe.Error = io.circe.DecodingFailure: Double

scala> val Xor.Left(e) = decode("\"NaN\"")(doubleWithFromString)
e: io.circe.Error = io.circe.DecodingFailure: Double

If "Infinty" and "NaN" are valid to be parsed as Double, one can fallback to using a Decoder[String].map(_.toDouble) approach.

[MartinSeeler]

I think this is a good idea!

[travisbrown]

Fixed in #189.