When mixing and enhancing objects don't attempt to assign keys to nul…

…l values. Should probably add more checks for undefined, etc.`

lib/core.js

@@ -37,6 +37,9 @@ var core = new (function () {
         }
         // If it's not a merge-copy, just set and forget
         else {
+          if (targ === null) {
+            continue;
+          }
           targ[p] = src[p];
         }
       }