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
```