Fix overly permissive parsing after whitespace (#59)

* Fix parsing logic after first whitespace

The way the parse was implemented accepted additional numeric characters
or `.` after the first valid `f64` literal but ignored them.

This permitted more strings that are invalid following a strict grammar
for byte sizes and silently ignores the further symbols without error.

```
1.0 ...KB
1.0 42.0 B
```
This change makes those illegal.

`1 000 B` was also subject to the bad `skip_while` ignoring the
following `000`. In this version of the fix whitespace is not accepted
as a digit separator. So it will raise an error if the user doesn't
explicitly strip the whitespace instead of reporting a wrong value.

* Add tests for invalid chars after whitespace

* Add test documenting that whitespace is not a valid digit separator

More following the old choices of using `f64::from_str`

---------

Co-authored-by: Rob Ede <robjtede@icloud.com>

src/parse.rs

@@ -10,9 +10,7 @@ impl std::str::FromStr for ByteSize {
         let number = take_while(value, |c| c.is_ascii_digit() || c == '.');
         match number.parse::<f64>() {
             Ok(v) => {
-                let suffix = skip_while(value, |c| {
-                    c.is_whitespace() || c.is_ascii_digit() || c == '.'
-                });
+                let suffix = skip_while(&value[number.len()..], char::is_whitespace);
                 match suffix.parse::<Unit>() {
                     Ok(u) => Ok(Self((v * u) as u64)),
                     Err(error) => Err(format!(
@@ -220,6 +218,11 @@ mod tests {
 
         assert!(parse("").is_err());
         assert!(parse("a124GB").is_err());
+        assert!(parse("1.3 42.0 B").is_err());
+        assert!(parse("1.3 ... B").is_err());
+        // The original implementation did not account for the possibility that users may
+        // use whitespace to visually separate digits, thus treat it as an error
+        assert!(parse("1 000 B").is_err());
     }
 
     #[test]