Add simple support for redirects

Follow 3XX status redirects with a new request. Parse the `location`
header for an absolute url, absolute path, or relative path. Replace or
amend the url from the original request and place a new request.

Convert all keys in the headers dictionary to lowercase to perform a
case-insensitive search.

There's no limit on the number of consecutive redirects. Since the
`requets()` method is now recursive, multiple redirects may crash the
stack. Especially on a CircuitPython microcontroller.

Expand the requests_simpletest_cpython.py example with new requests
utilizing the three supported types of redirects.

Note: We're using the httpbingo.org domain for redirects until
the following issue on httpbin.org is resolved:
postmanlabs/httpbin#617

This has been tested on CPython 3.8 and an ESP32-S2 based MagTag board
running CircuitPython 6.0.

Author: mogenson

adafruit_requests.py

@@ -281,18 +281,12 @@ def _parse_headers(self):
 
             content = self._readto(b"\r\n")
             if title and content:
-                title = str(title, "utf-8")
+                # enforce that all headers are lowercase
+                title = str(title, "utf-8").lower()
                 content = str(content, "utf-8")
-                # Check len first so we can skip the .lower allocation most of the time.
-                if (
-                    len(title) == len("content-length")
-                    and title.lower() == "content-length"
-                ):
+                if title == "content-length":
                     self._remaining = int(content)
-                if (
-                    len(title) == len("transfer-encoding")
-                    and title.lower() == "transfer-encoding"
-                ):
+                if title == "transfer-encoding":
                     self._chunked = content.strip().lower() == "chunked"
                 self._headers[title] = content
 
@@ -587,7 +581,27 @@ def request(
 
         resp = Response(socket, self)  # our response
         if "location" in resp.headers and 300 <= resp.status_code <= 399:
-            raise NotImplementedError("Redirects not yet supported")
+            # a naive handler for redirects
+            redirect = resp.headers["location"]
+
+            if redirect.startswith("http"):
+                # absolute URL
+                url = redirect
+            elif redirect[0] == "/":
+                # relative URL, absolute path
+                url = "/".join([proto, dummy, host, redirect[1:]])
+            else:
+                # relative URL, relative path
+                path = path.rsplit("/", 1)[0]
+
+                while redirect.startswith("../"):
+                    path = path.rsplit("/", 1)[0]
+                    redirect = redirect.split("../", 1)[1]
+
+                url = "/".join([proto, dummy, host, path, redirect])
+
+            self._last_response = resp
+            resp = self.request(method, url, data, json, headers, stream, timeout)
 
         self._last_response = resp
         return resp

examples/requests_simpletest_cpython.py

@@ -10,6 +10,9 @@
 TEXT_URL = "http://wifitest.adafruit.com/testwifi/index.html"
 JSON_GET_URL = "http://httpbin.org/get"
 JSON_POST_URL = "http://httpbin.org/post"
+REDIRECT_URL = "http://httpbingo.org/redirect/1"
+RELATIVE_REDIRECT_URL = "http://httpbingo.org/relative-redirect/1"
+ABSOLUTE_REDIRECT_URL = "http://httpbingo.org/absolute-redirect/1"
 
 print("Fetching text from %s" % TEXT_URL)
 response = http.get(TEXT_URL)
@@ -45,3 +48,26 @@
 # Parse out the 'json' key from json_resp dict.
 print("JSON Data received from server:", json_resp["json"])
 print("-" * 40)
+
+print("Fetching JSON data from redirect url %s" % REDIRECT_URL)
+response = http.get(REDIRECT_URL)
+print("-" * 40)
+
+print("JSON Response: ", response.json())
+print("-" * 40)
+
+print("Fetching JSON data from relative redirect url %s" % RELATIVE_REDIRECT_URL)
+response = http.get(RELATIVE_REDIRECT_URL)
+print("-" * 40)
+
+print("JSON Response: ", response.json())
+print("-" * 40)
+
+print("Fetching JSON data from aboslute redirect url %s" % ABSOLUTE_REDIRECT_URL)
+response = http.get(ABSOLUTE_REDIRECT_URL)
+print("-" * 40)
+
+print("JSON Response: ", response.json())
+print("-" * 40)
+
+response.close()