fix: populate GetBody to support 307/308 redirects

[dwisiswant0]

Description

When sending POST requests with a non-empty body
that trigger 307/308 redirects, the redirects fail
because the underlying http.Request.GetBody is
nil by default.

This patch populates GetBody using the existing
reusable body reader, allowing the net/http
client to rewind and replay the body.

Fixes #493.

Proof

patch:

$ go test -v -race -run "^TestRedirectPOST" .
=== RUN   TestRedirectPOSTWithBody
--- PASS: TestRedirectPOSTWithBody (0.00s)
=== RUN   TestRedirectPOSTWithBodyFromRequest
--- PASS: TestRedirectPOSTWithBodyFromRequest (0.00s)
PASS
ok  	github.com/projectdiscovery/retryablehttp-go	1.052s

main:

$ git cherry-pick e49953b
[main 9b73fba] test: adds `TestRedirectPOST*` tests
 Date: Sat Dec 6 00:04:02 2025 +0700
 1 file changed, 124 insertions(+)
$ go test -v -race -run "^TestRedirectPOST" .
=== RUN   TestRedirectPOSTWithBody
    request_test.go:105: Expected status 200, got 307. Body: 
--- FAIL: TestRedirectPOSTWithBody (0.00s)
=== RUN   TestRedirectPOSTWithBodyFromRequest
--- PASS: TestRedirectPOSTWithBodyFromRequest (0.00s)
FAIL
FAIL	github.com/projectdiscovery/retryablehttp-go	0.052s
FAIL

[dwisiswant0]

Superseded by #496