Inconsistent unescaping of URL parameters

I discovered some inconsistent unescaping of URL path parameters depending on the content of the parameter value:

- they are `url.PathUnescape`d if no `/` is contained
- otherwise the parameter value is still path escaped

This was unexpected for me since I could not find any documentation about how `chi` is handling the unescaping of URL parameters. I included a test case that demonstrates this issue for `chi@master` but the issue is also present for the latest release version. Here's the test output:

```sh
$ go test .
--- FAIL: TestURLParam (0.00s)
--- FAIL: TestURLParam/escaped_urlparam_containing_a_slash (0.00s)
go-chi-urlparam_test.go:27:
Error Trace: go-chi-urlparam_test.go:27
server.go:2042
mux.go:437
server.go:2042
mux.go:86
go-chi-urlparam_test.go:29
Error: Not equal:
expected: "'/"
actual : "%27%2F"

Diff:
--- Expected
+++ Actual
@@ -1 +1 @@
-'/
+%27%2F
Test: TestURLParam/escaped_urlparam_containing_a_slash
Messages: URLParam
FAIL
FAIL chi-urlparam-test 0.003s
FAIL
```

You can find the test files in this zip archive: [go-chi-urlparam-test.zip](https://github.com/go-chi/chi/files/5750881/go-chi-urlparam-test.zip)

For convenience this is the content of unit test file:

```go
package main

import (
"net/http"
"testing"

"github.com/go-chi/chi"
"github.com/stretchr/testify/assert"
)

func TestURLParam(t *testing.T) {
tCases := []struct {
name,
u,
expected string
}{
{"plain urlparam", "http://doesnot.matter/whatever", "whatever"},
{"escaped urlparam without slash", "http://doesnot.matter/%27", "'"},
{"escaped urlparam containing a slash", "http://doesnot.matter/%27%2F", "'/"},
}

for _, tCase := range tCases {
t.Run(tCase.name, func(t *testing.T) {
r := chi.NewRouter()
r.Get("/{value}", func(w http.ResponseWriter, r *http.Request) {
assert.Equal(t, tCase.expected, chi.URLParam(r, "value"), "URLParam")
})
assert.HTTPSuccess(t, r.ServeHTTP, "GET", tCase.u, nil)
})
}
}
```

Start by running the provided TestURLParam case with go test ., using go-chi-urlparam_test.go and its /{value} route as the reproduction. Trace URLParam handling from the router entry point to determine why values containing an encoded slash differ, then add or update regression coverage so the expected parameter value is consistent.

Written by the indexing model from the issue text.