ssl-proxies: Fix path validator when cert is lacking key usage extensions

270e41c6b59f5f113edd18cbad71956444f2090c introduced an enum to represent
key usage bits. The patch broke the key usage change for non-proxy certs
in that it did not deal correctly with the absence of the extension.
Merge pull request #92 from gbehrmann/fix/missing_key_usage_extension

ssl-proxies: Fix path validator when cert is lacking key usage extensions