strconv: fix on ftoa string decimal aproximation, 'nan' string fix

vlib/strconv/ftoa/f32_f64_to_string_test.v

@@ -165,4 +165,8 @@ fn test_float_to_str(){
 		//println(b)
 		assert b.len == exp + 2
 	}
+
+	// test rounding str conversion
+	assert f64_to_str(0.3456789123456, 4)=="3.4568e-01"
+	assert f32_to_str(0.345678, 3)=="3.457e-01"
 }