support for multiple header values. e.g. to set multiple cookies

[jbleijenbergh]

Need:
In some (load) scenarios multiple cookies should be captured and send (even if they contain a comma)

Spec

see https://www.rfc-editor.org/rfc/rfc7230#section-3.2.2

proposed changes:

• change the type for header definitions from string
  "headers": {"Content-Type": "application/json"
  to a list
  "headers": {"Content-Type": ["application/json"]
• when processing responses allow for multiple values for a header
• adjust replacement logic for tokens {{TOKEN_TO_REPLACE}} accordingly