request_value_semantics test failed

> [ RUN      ] message_test.request_value_semantics
> /home/nico159/cpp-netlib/http/test/request_test.cpp:42: Failure
> Value of: copy != assigned
>  Actual: false
>  Expected: true
> [  FAILED  ] message_test.request_value_semantics (28 ms)

The current implementation in request of swap and equals is:
```C++
  // Then we lift the swap and equals implementation.
  using request_base::swap;
  using request_base::equals;
```

But it's not good, as they don't care about request_pimpl in http::request
So I defined swap and equals for request (and also added an unit test for swap)

I'm a bit unsure if this it the correct way to fix the unit test