Bugfix: Ensure array-style 'set-cookie' headers are parsed correctly

[tomo324]

closes #287

• When an actualCookie is an array with multiple elements, they are joined with '; ' to make a single string before parsing.
  • It is OK without it, but adding ';' after all join() operations are complete ensures that the state before parsing multiple cookies is the same as a single string cookie.

Example:
If an array of cookies like ["one=1", "two=2"] is set via set-cookie header, the join() operation will result in a string like 'one=1; two=2;' which can be parsed correctly.