assert: handle []byte array properly

The regexp package works more efficiently on bytes; if you have bytes,
it is easier to pass these directly to assert.Regexp than to convert
them to a string first.

In addition, FindIndex/FindStringIndex are unnecessary here because we
immediately throw away the result - let's just call Match() instead.

assert/assertions.go

@@ -1615,15 +1615,21 @@ func ErrorContains(t TestingT, theError error, contains string, msgAndArgs ...in
 
 // matchRegexp return true if a specified regexp matches a string.
 func matchRegexp(rx interface{}, str interface{}) bool {
-
 	var r *regexp.Regexp
 	if rr, ok := rx.(*regexp.Regexp); ok {
 		r = rr
 	} else {
 		r = regexp.MustCompile(fmt.Sprint(rx))
 	}
 
-	return (r.FindStringIndex(fmt.Sprint(str)) != nil)
+	switch v := str.(type) {
+	case []byte:
+		return r.Match(v)
+	case string:
+		return r.MatchString(v)
+	default:
+		return r.MatchString(fmt.Sprint(v))
+	}
 
 }