Stringified object representations don't preserve field order about circe

[alexarchambault]

@longcao Order is preserved internally, it's just that the default printers don't preserve it in their output, see https://github.com/travisbrown/circe/blob/master/core/shared/src/main/scala/io/circe/Printer.scala#L215 - it's the same as in argonaut.

The following gives back the right order for me:

Printer.spaces2.copy(preserveOrder = true).pretty(m.asJson)

giving

res1: String = """
{
  "a" : 5,
  "b" : "asdf",
  "c" : [
    "zxcv",
    "qwer",
    "hjkl"
  ],
  "d" : false
}
"""

[travisbrown]

Fixed by #89—thanks @longcao, @alexarchambault!