[:bug:] Empty headers get mapped as "null"

[kirstein]

Type:
<:bug:>

Description:
If you have defined a HTTP header mapping for a lambda in agw and you do not provide the given HTTP header then the header will be mapped as "null".

Version: 2.9.6

Environment:
OSX, node 4.4.3

Steps to reproduce:

• Create service
• Create an endpoint
• Map header Xxx
• Make a curl request to the given endpoint with Xxx header

Expected result:
Header should be mapped as "" (thats the way AGW does it)

Actual result:
"null"

Affected features:

• AGW header mapping

[henrinormak]

This should've been fixed by 1a551c1