[hack] Fix printing of floats in Hh_json
[hack] Fix printing of floats in Hh_json

Summary: Per RFC4627, floats must have a digit following the decimal point, and `NaN` and infinite values are forbidden. Right now we print things like `1.` because that's what `string_of_float` does, but it would need to be either `1` or `1.0`.

Reviewed By: @nmote

Differential Revision: D2153646

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