More robust response datatype handling (#1674)

* HTTP1 header formatting moved to headers.format_headers and rewritten.

- New implementation is one line of code and twice faster than the old one.
- Whole header block encoded to UTF-8 in one pass.
- No longer supports custom encode method on header values.
- Cookie objects now have __str__ in addition to encode, to work with this.

* Linter

* format_http1_response

* Replace encode_body with faster implementation based on f-string.

Benchmarks:

def encode_body(data):
    try:
        # Try to encode it regularly
        return data.encode()
    except AttributeError:
        # Convert it to a str if you can't
        return str(data).encode()

def encode_body2(data):
    return f"{data}".encode()

def encode_body3(data):
    return str(data).encode()

data_str, data_int = "foo", 123

%timeit encode_body(data_int)
928 ns ± 2.96 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit encode_body2(data_int)
280 ns ± 2.09 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit encode_body3(data_int)
387 ns ± 1.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit encode_body(data_str)
202 ns ± 1.9 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

%timeit encode_body2(data_str)
197 ns ± 0.507 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

%timeit encode_body3(data_str)
313 ns ± 1.28 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

* Wtf linter

* Content-type fixes.

* Body encoding sanitation, first pass.
- body/data type autodetection fixed.
- do not repr(body).encode() bytes-ish values.
- support __html__ and _repr_html_ in sanic.response.html().

* <any type>-to-str response autoconversion limited to sanic.response.text() only.

* Workaround MyPy issue.

* Add an empty line to make isort happy.

* Add html test for __html__ and _repr_html_.

* Remove StreamingHTTPResponse.get_headers helper function.

* Add back HTTPResponse Keep-Alive removed by earlier merge or something.

* Revert "Remove StreamingHTTPResponse.get_headers helper function."

Tests depend on this otherwise useless function.

This reverts commit 9651e6ae01.

* Add deprecation warnings; instead of assert for wrong HTTP version, and for non-string response.text.

* Add back missing import.

* Avoid duplicate response header tweaking code.

* Linter errors

tests/test_requests.py

@@ -11,7 +11,7 @@ import pytest
 from sanic import Blueprint, Sanic
 from sanic.exceptions import ServerError
 from sanic.request import DEFAULT_HTTP_CONTENT_TYPE, Request, RequestParameters
-from sanic.response import json, text
+from sanic.response import html, json, text
 from sanic.testing import ASGI_HOST, HOST, PORT
 
 
@@ -72,6 +72,41 @@ def test_text(app):
     assert response.text == "Hello"
 
 
+def test_html(app):
+    class Foo:
+        def __html__(self):
+            return "<h1>Foo</h1>"
+
+        def _repr_html_(self):
+            return "<h1>Foo object repr</h1>"
+
+    class Bar:
+        def _repr_html_(self):
+            return "<h1>Bar object repr</h1>"
+
+    @app.route("/")
+    async def handler(request):
+        return html("<h1>Hello</h1>")
+
+    @app.route("/foo")
+    async def handler(request):
+        return html(Foo())
+
+    @app.route("/bar")
+    async def handler(request):
+        return html(Bar())
+
+    request, response = app.test_client.get("/")
+    assert response.content_type == "text/html; charset=utf-8"
+    assert response.text == "<h1>Hello</h1>"
+
+    request, response = app.test_client.get("/foo")
+    assert response.text == "<h1>Foo</h1>"
+
+    request, response = app.test_client.get("/bar")
+    assert response.text == "<h1>Bar object repr</h1>"
+
+
 @pytest.mark.asyncio
 async def test_text_asgi(app):
     @app.route("/")