Max size restriction applied to whole message not just data

For **incoming** messages the max size restriction is applied to the whole message, not just the data field:

https://github.com/libp2p/pull-mplex/blob/master/src/mplex.js#L84-L93

The [spec says that the data field can be up to 1MB](https://github.com/libp2p/specs/tree/master/mplex#writing-to-a-stream) so this will be causing false positives for certain message sizes.