Created methods to append and finish body content on Request (#1379)

* created methods to append and finish body content on request.py so the underlying body instance can have certain flexibility; modified server.py to reflect these changes

* - made some adjustments (including the Request.body_init method) as requested by @ahopkins;
- created a new test with a custom Request class implementation of the flexibility provided by the new methods;

tests/test_custom_request.py

@@ -0,0 +1,53 @@
+from io import BytesIO
+
+from sanic import Sanic
+from sanic.request import Request
+from sanic.response import json_dumps, text
+
+
+class CustomRequest(Request):
+    __slots__ = ("body_buffer",)
+
+    def body_init(self):
+        self.body_buffer = BytesIO()
+
+    def body_push(self, data):
+        self.body_buffer.write(data)
+
+    def body_finish(self):
+        self.body = self.body_buffer.getvalue()
+        self.body_buffer.close()
+
+
+def test_custom_request():
+    app = Sanic(request_class=CustomRequest)
+
+    @app.route("/post", methods=["POST"])
+    async def post_handler(request):
+        return text("OK")
+
+    @app.route("/get")
+    async def get_handler(request):
+        return text("OK")
+
+    payload = {"test": "OK"}
+    headers = {"content-type": "application/json"}
+
+    request, response = app.test_client.post(
+        "/post", data=json_dumps(payload), headers=headers
+    )
+
+    assert isinstance(request.body_buffer, BytesIO)
+    assert request.body_buffer.closed
+    assert request.body == b'{"test":"OK"}'
+    assert request.json.get("test") == "OK"
+    assert response.text == "OK"
+    assert response.status == 200
+
+    request, response = app.test_client.get("/get")
+
+    assert isinstance(request.body_buffer, BytesIO)
+    assert request.body_buffer.closed
+    assert request.body == b""
+    assert response.text == "OK"
+    assert response.status == 200