Fix wrong eth_type encoding in flow_mod

When checking flow mod prerequisites wrong value of eth_type was
used and every flow mod with eth_type match field was always discarded.
eth_type value should be encoded as binary, not integer.

Refs: #68
Add test flow mod to simple v4 controller

This flow mod matches on field ipv4_src=192.168.11.99 and changes field
ipv4_dst to 10.0.0.1 and outputs packet to port 2 of the switch.

Refs: #68