Made as_string(iterator) method of http_req_t::resource_t be private.
Removed version of resource_t::as_string that takes an iterator.
Improved http_req_t::resource_t exceptions, added ResourceTest.EmptyResource unit test.
Moved resource_test.cc to http_resource_test.cc.
Added a bunch of HttpResourceTest tests, documenting the behavior of http_req_t::resource_t.