Support multi-value headers in expect_header, expect_header_contains

[rfletcher]

Some response headers, like Set-Cookie, can be specified multiple times. That results in an array of values. Calling expect_header or expect_header_contains against one of these headers causes an exception:

Failure/Error: expect_header_contains(:set_cookie, 'foo')
NoMethodError:
  undefined method `downcase' for #<Array:0x007f9d811b64c8>

This patch adds support for headers with multiple values, checking the expectation against each value in the list and passing if any one value passes.