test: add test_use_UnixAdapter_directly

This new test tests using the `UnixAdapter` directly, like
[httpie-unixsocket](https://github.com/httpie/httpie-unixsocket) does.

I wrote this test because I found a case, on the `pluggable-urlparse`
branch, where tests were passing, but executing:

```
http http+unix://%2Fvar%2Frun%2Fdocker.sock/info
```

was failing.

requests_unixsocket/tests/test_requests_unixsocket.py

@@ -47,6 +47,20 @@ def get_sock_prefix(path):
 )
 
 
+def test_use_UnixAdapter_directly():
+    """Test using UnixAdapter directly, because
+    https://github.com/httpie/httpie-unixsocket does this
+    """
+    with UnixSocketServerThread() as usock_thread:
+        adapter = UnixAdapter()
+        request = requests.Request(
+            method='GET',
+            url='http+unix://%2Fvar%2Frun%2Fdocker.sock/info',
+        )
+        url = adapter.request_url(request=request, proxies=None)
+        assert url == '/info'
+
+
 def test_unix_domain_adapter_ok():
     with UnixSocketServerThread() as usock_thread:
         session = Session('http+unix://')