avoid mutating request headers in-place (spinframework#113)

Previously, the `spin_sdk.http.send` function automatically added a
`content-length` header to the header dictionary supplied by the caller if one
was not already present and if a request body was specified.  However, since we
were mutating the dictionary in-place, the change was visible to the caller.  In
addition to being potentially surprising to the caller, that can lead to issues
if the same object is reused for another request later, at which point the
header we added earlier will still be present but not necessarily accurate for
the new request.

To avoid such confusion, we now make a copy and add the `content-length` header
to the copy.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

src/spin_sdk/http/__init__.py

@@ -164,13 +164,20 @@ async def send_async(request: Request) -> Response:
             case _:
                 scheme = Scheme_Other(url_parsed.scheme)
 
-    if request.headers.get('content-length') is None:
+    headers_dict = request.headers
+
+    # Add a `content-length` header if the caller didn't include one, but did
+    # specify a body:
+    if headers_dict.get('content-length') is None:
         content_length = len(request.body) if request.body is not None else 0
-        request.headers['content-length'] = str(content_length)
+        # Make a copy rather than mutate in place, since the caller might not
+        # expect us to mutate it:
+        headers_dict = headers_dict.copy()
+        headers_dict['content-length'] = str(content_length)
 
     headers = list(map(
         lambda pair: (pair[0], bytes(pair[1], "utf-8")),
-        request.headers.items()
+        headers_dict.items()
     ))
 
     outgoing_request = OutgoingRequest(Fields.from_list(headers))