gh-59598: Ignore leading whitespace in JSONDecoder.raw_decode

Whitespace is allowed before JSON objects according to RFC 4627.

Lib/json/decoder.py

@@ -341,23 +341,27 @@ def decode(self, s, _w=WHITESPACE.match):
         containing a JSON document).
 
         """
-        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
+        obj, end = self.raw_decode(s)
         end = _w(s, end).end()
         if end != len(s):
             raise JSONDecodeError("Extra data", s, end)
         return obj
 
-    def raw_decode(self, s, idx=0):
+    def raw_decode(self, s, idx=0, _w=WHITESPACE.match):
         """Decode a JSON document from ``s`` (a ``str`` beginning with
         a JSON document) and return a 2-tuple of the Python
         representation and the index in ``s`` where the document ended.
+        Whitespace at the beginning of the document will be ignored.
+
+        Optionally, ``idx`` can be used to specify an offset in ``s``
+        where the document begins.
 
         This can be used to decode a JSON document from a string that may
         have extraneous data at the end.
 
         """
         try:
-            obj, end = self.scan_once(s, idx)
+            obj, end = self.scan_once(s, idx=_w(s, idx).end())
         except StopIteration as err:
             raise JSONDecodeError("Expecting value", s, err.value) from None
         return obj, end