findRule

```
benchmark                         old ns/op     new ns/op     delta
BenchmarkParseFromStatus401-4     23.9          23.8          -0.42%
BenchmarkParseFromString002-4     311424        310809        -0.20%
BenchmarkParseFromString001-4     86707         86562         -0.17%
```

robotstxt.go

@@ -208,7 +208,7 @@ func (g *Group) findRule(path string) (ret *rule) {
 				// From Google's spec:
 				// The order of precedence for rules with wildcards is undefined.
 				if l := len(r.pattern.String()); l > prefixLen {
-					prefixLen = len(r.pattern.String())
+					prefixLen = l
 					ret = r
 				}
 			}