Fix order-dependent Gitaly specs failing

If `spec/tasks/gitlab/cleanup_rake_spec.rb` preceded any of the Gitaly
request specs, it would import the `cleanup.rake` and the global
function `limit`. For some reason, the Protobuf implementation would use
the global function instead of the getter method. For example:

```
def limit
  puts "hi"
end

req = Gitaly::WikiGetAllPagesRequest.new
req.send(:limit)
hi
=> nil
```

To fix this problem, access the field value using the [] operator
instead.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64006

spec/support/matchers/gitaly_matchers.rb

@@ -9,6 +9,6 @@ end
 
 RSpec::Matchers.define :gitaly_request_with_params do |params|
   match do |actual|
-    params.reduce(true) { |r, (key, val)| r && actual.send(key) == val }
+    params.reduce(true) { |r, (key, val)| r && actual[key.to_s] == val }
   end
 end