Improve the error message when a header value does not match the assertion

[adrianiftode]

In this example, the header actually exists, but the error message suggests it doesn't. A better message would express the fact the value differs.

Expected value to contain the HTTP header "Location" having value "/api/iab/optout", but no such header was found in the actual response. 
 
 The HTTP response was:

HTTP/1.1 302 Redirect
Vary: Cookie
Location: /api/iab/status
Set-Cookie: XSRF-TOKEN=Fpxq1CaquEChrslAeZ37TAa; path=/
Content-Length: 0

***** Content is of a stream type having the length 0. *****

The originated HTTP request was:

GET https://.../api/iab/coop?action=3 HTTP 2.0
Cookie: 
Content-Length: 0