tests: set all fetch() params in test_redirect_limit

The default values of body and headers ought to be "nil", but in fact
they end up as the string "0" in test_redirect_limit.

This causes all sorts of problems. When body is defined as "0", we
follow the conditional that leads us to conn.request_post instead of
conn.request_get, and request_post crashes since it expects a key-value
string instead of simply "0". The unexpected string for the headers
variable wreaks havoc as well; when headers is "0", the hash value
assignment at the beginning of fetch() fails, because Ruby can't convert
the string to a hash.

I'm not sure why these values are "0" instead of nil, but setting them
explicitly during the call fixes the bug.

Author: ktdreyer

test/test_fetchers.rb

@@ -244,7 +244,7 @@ def test_redirect_limit
     @_redirect_counter = 0
     uri = _uri_build('/redirect_loop')
     assert_raises(OpenID::HTTPRedirectLimitReached) {
-      @fetcher.fetch(uri, redirect_limit=0)
+      @fetcher.fetch(uri, body=nil, headers=nil, redirect_limit=0)
     }
   end