Disallow constructing rowset from a session

This doesn't make sense, so explicitly delete the ctor from session to
prevent such code from compiling.

Closes #1081.

See #1082.

include/soci/rowset.h

@@ -182,6 +182,12 @@ class rowset
         pimpl_->incRef();
     }
 
+    // Due to the existence of conversion from session to prepare_temp_type, it
+    // would have been possible to construct a rowset from session if we didn't
+    // delete this ctor -- so do delete it because it doesn't make sense to
+    // provide such constructor.
+    rowset(session const& session) = delete;
+
     ~rowset()
     {
         pimpl_->decRef();