rafthttp: fix the misformat logging line

before:
```
2015/06/12 20:06:19 rafthttp: dropped MsgApp from %!s(uint64=2) since
receiving buffer is full
```

after:
```
2015/06/12 13:51:38 rafthttp: dropped MsgProp from 2 since receiving
buffer is full
```

rafthttp/stream.go

@@ -360,9 +360,9 @@ func (cr *streamReader) decodeLoop(rc io.ReadCloser, t streamType) error {
 			case recvc <- m:
 			default:
 				if cr.status.isActive() {
-					plog.Warningf("dropped %s from %s since receiving buffer is full", m.Type, m.From)
+					plog.Warningf("dropped %s from %s since receiving buffer is full", m.Type, types.ID(m.From))
 				} else {
-					plog.Debugf("dropped %s from %s since receiving buffer is full", m.Type, m.From)
+					plog.Debugf("dropped %s from %s since receiving buffer is full", m.Type, types.ID(m.From))
 				}
 			}
 		}