using acquireBuffer

reusing a buffer from the pool decreases the allocs and memory usage

```shell
% go test -benchmem -bench=. -run=^Benchmark
goos: darwin
goarch: amd64
pkg: github.com/go-resty/resty/v2
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
Benchmark_parseRequestURL_PathParams-16           753834              1367 ns/op             256 B/op          5 allocs/op
Benchmark_parseRequestURL_QueryParams-16         1000000              1167 ns/op             352 B/op          9 allocs/op
PASS
ok      github.com/go-resty/resty/v2    2.373s
```

middleware.go

@@ -54,11 +54,9 @@ func parseRequestURL(c *Client, r *Request) error {
 		}
 
 		if len(params) > 0 {
-			var (
-				prev int
-				buf  bytes.Buffer
-			)
-			buf.Grow(len(r.URL))
+			var prev int
+			buf := acquireBuffer()
+			defer releaseBuffer(buf)
 			// search for the next or first opened curly bracket
 			for curr := strings.Index(r.URL, "{"); curr > prev; curr = prev + strings.Index(r.URL[prev:], "{") {
 				// write everything form the previous position up to the current