test: cover both fail/recover redis connect cases

```
$ make busted-util
/usr/local/openresty/luajit/bin/rover exec bin/busted "spec/threescale_utils_spec.lua"
●●●
3 successes / 0 failures / 0 errors / 0 pending : 0.022514 seconds
```

gateway/src/apicast/threescale_utils.lua

@@ -194,7 +194,7 @@ function _M.error(resilient, ...)
   else
     ngx.status = ngx.HTTP_INTERNAL_SERVER_ERROR
     ngx.say(...)
-    if not resilient then
+    if not resilient or resilient == nil then
       ngx.exit(ngx.status)
     end
   end

spec/threescale_utils_spec.lua

@@ -11,13 +11,26 @@ describe('3scale utils', function()
             assert.equal('one two three', error)
         end)
 
+        it('fails nginx chain when redis cannot be connected', function()
+            stub(ngx, 'get_phase', function() return 'init' end)
+            stub(ngx, 'say', function(...) return nil end)
+            local exit = spy.on(ngx, 'exit', function(s) return 'exited!' end)
+
+            local error = _M.error(false, 'cache issue ' .. 'host:' .. 6379)
+
+            assert.spy(ngx.exit).was_called(1)
+            assert.spy(ngx.say).was.called_with('cache issue ' .. 'host:' .. 6379)
+        end)
+
         it('does not terminate with nginx.exit if resilient flag is set', function()
-            local exit = spy.on(ngx, 'exit', function() return 'exited!' end)
-            local error = _M.error(true, 'redis is not reachable')
+            stub(ngx, 'get_phase', function() return 'init' end)
+            stub(ngx, 'say', function(...) return nil end)
 
-            assert.spy(exit).was_not_called()
+            local exit = spy.on(ngx, 'exit', function(s) return 'exited!' end)
+            local error = _M.error(true, 'redis is not reachable')
 
-            assert.equal('redis is not reachable', error)
+            assert.spy(ngx.exit).was_not_called()
+            assert.spy(ngx.say).was.called_with('redis is not reachable')
         end)
     end)
 end)