fix(router) preserve_host always fetches the request's Host

Summary
-------

It was reported in #2825 that `preserve_host` seemed to behave wrongly
upon subsequent call of a same API if that API had no `hosts`.

Since matches APIs are cached, and so are their `upstream_host` values,
I investigated around the LRU cache of our matches in the router. What I
discovered is, considering the following API:

    {
        "name": "example",
        "upstream_url": "http://httpbin.org",
        "uris": ["/example"]
    }

The router would try to save cycles by *not* reading the
`ngx.var.http_host` variable, since no API is configured to match on a
Host header.

However, the Host is also part of the cache key since the introduction
of the LRU cache in the router. But in the above case, considering the
following two requests:

    GET /example HTTP/1.1
    Host: domain1.com

    GET /example HTTP/1.1
    Host: domain2.com

Both matches would be cached under the **same** key (since the
`req_host` argument of `find_api` is `nil` because the router does not
read the Host header):

    GET:/example:

Notice the lack of Host part after the last colon.

Hence, once this API would be matched after the first request, its
cached `upstream_host` would be `domain1.com`, and subsequent calls with
`Host: domain2.com` would hit the same key, and yield the same
`upstream_host` as the first request.

The approach taken by this fix is to **always** retrieve the Host
header. This choice is driven by the fact that cache keys should have as
high a cardinality as possible, and to ensure we avoid such duplicate
keys in the future.

Changes
-------

* remove `grab_header` optimization from `router.lua`
* remove `grab_header` tests from unit test suite
* make `find_api()`'s 3rd argument (Host header) mandatory
* update related unit tests to reflect the above change
* new regression test for this issue

---

Fix #2824

kong/core/router.lua

@@ -15,7 +15,6 @@ local tonumber = tonumber
 local ipairs   = ipairs
 local pairs    = pairs
 local type     = type
-local next     = next
 local band     = bit.band
 local bor      = bit.bor
 local ERR      = ngx.ERR
@@ -588,17 +587,14 @@ function _M.new(apis)
   end)
 
 
-  local grab_host = #wildcard_hosts > 0 or next(plain_indexes.hosts) ~= nil
-
-
   local function find_api(req_method, req_uri, req_host, ngx)
     if type(req_method) ~= "string" then
       return error("arg #1 method must be a string")
     end
     if type(req_uri) ~= "string" then
       return error("arg #2 uri must be a string")
     end
-    if req_host and type(req_host) ~= "string" then
+    if type(req_host) ~= "string" then
       return error("arg #3 host must be a string")
     end
 
@@ -831,13 +827,7 @@ function _M.new(apis)
       end
     end
 
-    --print("grab host header: ", grab_host)
-
-    local req_host
-
-    if grab_host then
-      req_host = ngx.var.http_host
-    end
+    local req_host = ngx.var.http_host
 
     local match_t = find_api(method, uri, req_host, ngx)
     if not match_t then