match_body: a secret after a body miss is never swapped #280

swapBody reads the body to EOF. A hit replaces it with a fresh buffer, but a miss leaves the BufferedBody at EOF, and the pipeline only rewinds between transforms. Every later match_body secret in the same transform reads an empty body, and its placeholder goes upstream unchanged.

Repro (v0.49.0, same code on main): three match_body secrets on one host, in the order proxy-username, proxy-notes, proxy-password. POST {"userName":"proxy-username","password":"proxy-password"}. The upstream gets the real username and the literal proxy-password, and the audit's swapped lists only the username secret.

Fix: on a miss, put the bytes that were read back as the body, as the hit path does. PR: #281