pg-protocol: Narrow type of BufferReader.encoding

`BufferReader.encoding` to `BufferEncoding` from `string` to match the new
signature of `Buffer.toString`.

Author: hyperair

packages/pg-protocol/src/buffer-reader.ts

@@ -2,7 +2,7 @@ export class BufferReader {
   private buffer: Buffer = Buffer.allocUnsafe(0)
 
   // TODO(bmc): support non-utf8 encoding?
-  private encoding: string = 'utf-8'
+  private encoding: BufferEncoding = 'utf-8'
 
   constructor(private offset: number = 0) {}