MINOR: mux-h2: Use <body_len> H2S field for payload without content-length

MINOR: mux-h2: Use <body_len> H2S field for payload without content-length

Before, the <body_len> H2S field was only use for verity the annonced
content-lenght value was respected. Now, this field is used for all
messages. Messages with a content-length are still handled the same way.
<body_len> is set to the content-length value and decremented by the size of
each DATA frame. For other messages, the value is initialized to ULLONG_MAX
and still decremented by the size of each DATA frame. This change is
mandatory to properly define the known input payload length value of the
sedesc.