isEmpty is being placed in headers

I found an issue where "isEmpty" is being written to the headers at line 278 in needle.js and does not happen 100% of the time. I am using the Node.js openwhisk package that calls needle for requests.

  // now that all our headers are set, overwrite them if instructed.
  for (var h in options.headers)
    config.headers[h.toLowerCase()] = options.headers[h];

In issue #167 the above lines of code are committed as d227ef4205809084fe085d74f793813ec644f168

I am not really sure how to repro this without showing confidential code and using company resources, but I think it should be as simple as checking the property type before adding values from option.headers to config.headers?

option.headers contains 3 properties at this point in the code:
Authorization
User-Agent
x-namespace-id

config.headers contains 1 property:
accept

As the loop progresses, all 3 of the option.headers properties are written to config.headers, but on the next loop isEmpty is added to config.headers causing the request to fail with the following message:

"TypeError [ERR_INVALID_CHAR]: Invalid character in header content ["isempty"]
    at ClientRequest.setHeader (_http_outgoing.js:474:3)
    at new ClientRequest (_http_client.js:184:14)
    at Object.request (https.js:272:10)
    at Needle.Object.<anonymous>.Needle.send_request (/MyProject/node_modules/needle/lib/needle.js:478:26)
    at next (/MyProject/node_modules/needle/lib/needle.js:371:10)
    at Needle.Object.<anonymous>.Needle.start (/MyProject/node_modules/needle/lib/needle.js:374:17)
    at Function.Object.<anonymous>.module.exports.request (/MyProject/node_modules/needle/lib/needle.js:794:56)
    at /MyProject/node_modules/needle/lib/needle.js:753:22
    at new Promise (<anonymous>)
    at Object.<anonymous>.module.exports (/MyProject/node_modules/needle/lib/needle.js:752:12)"

Watched Variables in the debugger at the 4th loop:

Start at needle.js line 278 and inspect how options.headers is iterated and how config.headers is built; compare the referenced commit d227ef4205809084fe085d74f793813ec644f168 with the current behavior. Find a reproducible case or regression test showing that isEmpty must not become an HTTP header, then verify the request succeeds without the invalid header.