Fix #914.

[bsansouci]

This is a pretty simple fix.
I use the existent parameter withStringKeys to when to format the key
as an ident, or as a simple string.

For now we don't support Ldot and Lapply in the string keys. I'm not sure what the behavior should be in this case.

cc @chenglou

[chenglou]

cc @yunxing

[chenglou]

every weird string key should be supported though; Is that hard? I believe the same will apply for #941

[bsansouci]

@chenglou I think Ldot and Lapply are kinds of identifier. In this case, we parse the key as an Lident https://github.com/facebook/reason/blob/c2661cabc1405b373a84047a61afffd0e121aef2/src/reason_parser.mly#L3483. So there's no way (right?) to have anything else in those case. That'll support any string (because we parse any string).

[yunxing]

Accidentally lost track on this, looking into it now.

[yunxing]

Very sorry for the delay. Diff looks good. Let's merge this!