Return early from removeContentHeaders if res._headers is null

[laktek]

Not sure if it's useful since this is a private API method. But I happened to invoke with headers set to null from another project.

[dougwilson]

This seems like a good idea since res._headers comes from node code and it only gets defined when headers get set, so there should be some guard in that method to keep it from blowing up.

[dougwilson]

@laktek Can you put that space back on the first line and squash into the original commit? Having the blank line is intentional.

[jonathanong]

haha that space is such a silly thing to care about :P

[laktek]

@dougwilson done.