Fix media_type_params when Content-Type parameters contains quoted-strings

According RFC 2616, the Content-Type parameters value can be even a token or
a quoted-string:

    media-type     = type "/" subtype *( ";" parameter )
    type           = token
    subtype        = token
    parameter      = attribute "=" value
    attribute      = token
    value          = token | quoted-string
    quoted-string  = ( <"> *(qdtext | quoted-pair ) <"> )
    qdtext         = <any TEXT except <">>"
    quoted-pair    = "\" CHAR