Request.Marshal() drains the body and drops ProxyURL / ResponseCharacterEncoding

[Noethix55555]

Summary

Request.Marshal() does not round-trip a request faithfully. Two distinct facets:

Facet A: Marshal() destroys the original body (non-idempotent)

In Marshal(), the body is read with io.ReadAll(r.Body) but never restored:

if r.Body != nil {
    body, err = io.ReadAll(r.Body)
    ...
}

After Marshal() returns, r.Body is fully consumed. As a result:

• Calling Marshal() a second time produces a request with an empty body.
• Calling Do() or Retry() after a Marshal() sends an empty body.

Facet B: ProxyURL and ResponseCharacterEncoding are silently dropped

The internal serializableRequest struct omits ProxyURL and ResponseCharacterEncoding, so Marshal() -> UnmarshalRequest() loses them. ResponseCharacterEncoding is a user-set, pre-fetch field consumed by fixCharset; losing it on a queued or persisted request changes behavior (silently falls back to auto-detect).

Reproduction

• Marshal the same request twice -> the second result has "Body":"".
• Set ProxyURL / ResponseCharacterEncoding, Marshal then UnmarshalRequest -> both come back empty.

Fix

Restore r.Body after reading, and add ProxyURL + ResponseCharacterEncoding to serializableRequest (populated in Marshal, restored in UnmarshalRequest). PR incoming.