The headers field in func.yaml expects an array of values for each header key

[treeder]

HTTP spec says values split by commas:

• http://stackoverflow.com/questions/3096888/standard-for-adding-multiple-values-of-a-single-http-header-to-a-request-or-resp
• https://www.tutorialspoint.com/http/http_header_fields.htm

Let's just have it take a single key/value in the yaml. If user wants multiple, they can add the commas.