Values in header will not be overwritten when specified in custom values file

[IncredibleHannes]

I was trying to generate tests for the 415 unsupported mediatype status code.
In order to specify an mediatype that is not supported I used a custom value file containing the following definition:
"415": { "header": { "Accept": "undefined" }
The Accept field will be overwritten by the test generation. It works other keys that won't be automatically generated.

I already fixed this bug and I will contribute a PR.

[noahdietz]

Thanks for the fix! Much appreciated.