map: invalid keys sort

@achille-roussel Hello,

As I was reading the code of the package, I noticed the sortKeys function that is created based on the map key type to encode, and saw that you sort on the reflect.Value value rather than the string representation of the key.

I was perplex to find that as an optin functionality, since the package aims to be compliant with the standard library, unless I misunderstood the README.

The following example shows that the output is different when using a map with keys of type int: https://play.golang.org/p/1ddbUssdbaf

The only reference to a map with keys of signed/unsigned-integer kind I could find in the tests is at

encoding/json/json_test.go

Line 190 in bc545e9

map[int]bool{1: false, 42: true},

but the keys are already lexicographicaly sorted.

@wI2L Thanks for reporting the issue! And you're correct, this seems like a mistake.

Would you be available to submit a fix? Otherwise I'll take a look at it sometimes this weekend.

@achille-roussel Sure, I'll send a PR tomorrow, this is a quickfix.

Any update?

I'm working on it.