lib/json: encode floating point numbers with a decimal point

The previous code carelessly used Go's %g operator
when it should have used Starlark's.

Fixes #563

lib/json/json.go

@@ -147,7 +147,8 @@ func encode(thread *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, k
 			if !isFinite(float64(x)) {
 				return fmt.Errorf("cannot encode non-finite float %v", x)
 			}
-			fmt.Fprintf(buf, "%g", x) // always contains a decimal point
+			// Float.String always contains a decimal point. (%g does not!)
+			buf.WriteString(x.String())
 
 		case starlark.String:
 			quote(string(x))

starlark/testdata/json.star

@@ -144,8 +144,13 @@ numbers = [
     0, 1, -1, +1, 1.23e45, -1.23e-45,
     3539537889086624823140625,
     float(3539537889086624823140625),
+    float(1.0),
 ]
-assert.eq(codec(numbers), numbers)
+def number_roundtrip():
+    show = lambda n: (n, type(n))
+    for n in numbers:
+      assert.eq(show(codec(n)), show(n)) # ensure no int/float confusion
+number_roundtrip()
 
 ## json.indent