Allow unsetting headers by passing null (#382) (#1845)

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/utils.js

@@ -294,6 +294,11 @@ function deepMerge(/* obj1, obj2, obj3, ... */) {
     }
   }
 
+  var lastArgument = arguments[arguments.length - 1];
+  if (lastArgument === null || typeof lastArgument === 'undefined') {
+    return lastArgument;
+  }
+
   for (var i = 0, l = arguments.length; i < l; i++) {
     forEach(arguments[i], assignValue);
   }