wsgi: Raise TooLarge even when Content-Length is not provided.

Prevent the (very unlikely at a 10MB given the manipulated data structures)
risk of a partial read accidentally containing producing a well-formed
result.
Also, only accept base-10 content lengths.

caucase/test.py

@@ -2241,6 +2241,12 @@ class CaucaseTest(unittest.TestCase):
       'CONTENT_LENGTH': str(wsgi.MAX_BODY_LENGTH + 1),
       'wsgi.input': BytesIO(),
     })[0], 413)
+    self.assertEqual(request({
+      'PATH_INFO': '/cau/crt/renew',
+      'REQUEST_METHOD': 'PUT',
+      'CONTENT_TYPE': 'application/json',
+      'wsgi.input': BytesIO(b'"' + b'a' * (wsgi.MAX_BODY_LENGTH + 1)),
+    })[0], 413)
     self.assertEqual(request({
       'PATH_INFO': '/cau/crt/renew',
       'REQUEST_METHOD': 'PUT',

caucase/wsgi.py

@@ -652,8 +652,14 @@ class Application(object):
     Raises TooLarge if Content-Length if over MAX_BODY_LENGTH.
     If Content-Length is not set, reads at most MAX_BODY_LENGTH bytes.
     """
+    content_length = environ.get('CONTENT_LENGTH')
+    if not content_length:
+      result = environ['wsgi.input'].read(MAX_BODY_LENGTH)
+      if environ['wsgi.input'].read(1):
+        raise TooLarge(b'Content-Length limit exceeded')
+      return result
     try:
-      length = int(environ.get('CONTENT_LENGTH') or MAX_BODY_LENGTH)
+      length = int(content_length, 10)
     except ValueError:
       raise BadRequest(b'Invalid Content-Length')
     if length > MAX_BODY_LENGTH: