api/dcim/interface-connections allows duplicate connections via opposing directionality

[lampwins]

Issue type: bug report

Python version: 2.7
NetBox version: 2.0.7

Given two interfaces, create two connections by reversing interface_a and interface_b. The api allows this, thus creating the same connection twice but in different directions. I would expect the second request to fail saying one or both of these interfaces is already connected.

curl -X POST  -d '{ \ 
   "interface_b": "1970", \ 
   "interface_a": "1945" \ 
 }' 'http://netbox-dev.nw.cougars.int/api/dcim/interface-connections/'

curl -X POST -d '{ \ 
   "interface_b": "1945", \ 
   "interface_a": "1970" \ 
 }' 'http://netbox-dev.nw.cougars.int/api/dcim/interface-connections/'

[jeremystretch]

This stems from the same root issue as #1285 (lack of model validation via the API). I'm going to close this issue to concentrate any discussion on the matter there.