Detect multiple boundary values

[ErikWegner]

Run into a stuck multipart parsing (in axum), because a request had two boundaries within the header like this:

Content-Type: multipart/form-data; boundary=---------------------------17832986845363619572950809769; boundary=------------------------bjsYDym0V6iqcnC7ILHUdu

The parser used the first value and never succeeded in finding a part, it did even blocked the whole connection.

This change detects multiple values for boundary and prevents a dead lock.