CONTINUOUS should be decoded depending on the first frame

**Describe the bug**
Due to changes with #1165 a return was introduced to handle not decode messages where no RSV1 is set.
This is however the case for a frame with opcode CONTINUOUS.
It is a valid framing and should therefore work

**To Reproduce**
Steps to reproduce the behavior:
1. Run autobahn testsuite 

**Expected behavior**
autoban testsuite passes

