Preserve individual headers with the same name on responses (#1208)

* Preserve individual headers with the same name on responses

* Rename RequestHeadersDict to HTTPHeadersDict

* Update tests/utils/http_server.py

* Update tests/utils/http_server.py

* Update httpie/adapters.py

Co-authored-by: Jakub Roztocil <jakub@roztocil.co>

tests/utils/http_server.py

@@ -0,0 +1,50 @@
+import threading
+
+from collections import defaultdict
+from http import HTTPStatus
+from http.server import HTTPServer, BaseHTTPRequestHandler
+from urllib.parse import urlparse
+
+import pytest
+
+
+class TestHandler(BaseHTTPRequestHandler):
+    handlers = defaultdict(dict)
+
+    @classmethod
+    def handler(cls, method, path):
+        def inner(func):
+            cls.handlers[method][path] = func
+            return func
+        return inner
+
+    def do_GET(self):
+        parse_result = urlparse(self.path)
+        func = self.handlers['GET'].get(parse_result.path)
+        if func is None:
+            return self.send_error(HTTPStatus.NOT_FOUND)
+
+        return func(self)
+
+
+@TestHandler.handler('GET', '/headers')
+def get_headers(handler):
+    handler.send_response(200)
+    for key, value in handler.headers.items():
+        handler.send_header(key, value)
+    handler.send_header('Content-Length', 0)
+    handler.end_headers()
+
+
+@pytest.fixture(scope="function")
+def http_server():
+    """A custom HTTP server implementation for our tests, that is
+    built on top of the http.server module. Handy when we need to
+    deal with details which httpbin can not capture."""
+
+    server = HTTPServer(('localhost', 0), TestHandler)
+    thread = threading.Thread(target=server.serve_forever)
+    thread.start()
+    yield '{}:{}'.format(*server.socket.getsockname())
+    server.shutdown()
+    thread.join(timeout=0.5)