buffer: write() should always set _charsWritten.

Fixes #1633.

lib/buffer.js

@@ -122,6 +122,7 @@ SlowBuffer.prototype.hexWrite = function(string, offset, length) {
     if (isNaN(byte)) throw new Error('Invalid hex string');
     this[offset + i] = byte;
   }
+  SlowBuffer._charsWritten = i * 2;
   return i;
 };
 

test/simple/test-buffer.js

@@ -687,3 +687,5 @@ buf.write('0123456789', 'binary');
 assert.equal(Buffer._charsWritten, 9);
 buf.write('123456', 'base64');
 assert.equal(Buffer._charsWritten, 6);
+buf.write('00010203040506070809', 'hex');
+assert.equal(Buffer._charsWritten, 18);