When Undertow receives a request in which the first header line begins
with one or more spaces, it strips them before processing the request.
This is usable as a request smuggling primitive.

The HTTP RFCs state that when a field-line begins with a space or tab,
it is permissible to concatenate it into the previous field-line's
value. This is referred to as `obs-fold` in the RFCs. However, it is
always invalid to obs-fold on the first line, since there is no
previous field-line to concatenate into. Thus, the message should be
rejected.