Ethereum Yellow Paper states:
nonce: A scalar value equal to the number of transactions sent from this address or, in the case of accounts with associated code, the number of contract-creations made by this account. For account of address a in state σ, this would be formally denoted σ[a]n.
This statement does not describe situation where there is a private key for address with code. Also, I haven't found any statements to forbid existence of private key for a contract. Intuition says that in both cases nonce just incremented but I need to know this for sure from an authorized source.