poor error message on malformed media range

[GoogleCodeExporter]

I'm seeing this Accept header in the wild: 

`text/html,application/json;q=0.9application/xhtml+xml,application/xml;q=0.9,*/*
;q=0.8`

Note the `0.9application`, which I believe is an error and should be 
`0.9,application`. mimeparse.parse_media_range fails with `ValueError: invalid 
literal for float(): 0.9application/xhtml+xml`. I would expect instead an error 
message saying the header is malformed. Does mimeparse have a concept of strict 
and lenient parsing? Does it want one?

Original issue reported on code.google.com by whit537@gmail.com on 20 May 2013 at 3:44

[GoogleCodeExporter]

Relatedly, it chokes on an Accept header (again, found in the wild) of:

'*/*;'

which again is arguably malformed.  So should mimeparse be doing its best to 
cope with these found-in-the-wild errors?  Or should it be throwing some kind 
of 'Malformed Header' exception? (So a server using it could raise a 400, for 
example?)

Original comment by pjimen...@gmail.com on 8 Oct 2013 at 3:52