initialize headers to {} if we need to copy some. 

Not sure if this is a bug in underscore, but without this I got: 

najax/node_modules/underscore/underscore.js:822
          obj[prop] = source[prop];
                    ^
TypeError: Cannot set property '...' of undefined
saw a bit about possibly setting other headers

decided that it was a better fix to only set the destination headers to an empty if they were undefined and there were destination headers.
Merge pull request #2 from rhildred/master

initialize headers to {} if we need to copy some.