Changed encode_number() number->string conversion method

Using Lua5.3, tonumber() will represent the float `1` as "1.0" instead
of "1". Using string.format("%14g, x) `1` is converted to "1" regardless
of whether it's an int or float. All other conversions seem to be
uneffected.

json.lua

@@ -83,7 +83,7 @@ local function encode_number(val)
   if val ~= val or val <= -math.huge or val >= math.huge then
     error("unexpected number value '" .. tostring(val) .. "'")
   end
-  return tostring(val)
+  return string.format("%.14g", val)
 end