fix a small bug with load_publickey (#481)

* fix a small bug with load_publickey

* update docstring, rename test method

* make hynek happy
diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 21bdadd..4c1ce87 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -2450,6 +2450,7 @@
 
     pkey = PKey.__new__(PKey)
     pkey._pkey = _ffi.gc(evp_pkey, _lib.EVP_PKEY_free)
+    pkey._only_public = True
     return pkey