Add protection against concurrent Readers

During my use of the library(server and client both v1.8.7), the server occasionally receives the following prompt,

```
failed to get reader: received header with unexpected rsv bits set: false:true:true
```

which seems to be related to this part of the code.

**write.go** L300:

```
c.writeHeader.rsv1 = false
if flate && (opcode == opText || opcode == opBinary) {
c.writeHeader.rsv1 = true
}
```

Could you please confirm if it is necessary to add the following code here?

```
c.writeHeader.rsv1 = false
c.writeHeader.rsv2 = false
c.writeHeader.rsv3 = false
```

Start in write.go around line 300 and trace how writeHeader.rsv1, rsv2, and rsv3 are set while concurrent readers or writers are active. Use the reported unexpected RSV-bits error as the failure signal. Done means the concurrent-reader scenario no longer produces invalid RSV bits, with a regression test covering the behavior if the repository has an appropriate test entry point.

索引モデルが issue の本文から書いたものです。