Honour the charset that comes with content/type=...;charset=foobar header when producing a HAR

Hiya!

We stumbled into a problem when trying to get HARs that included csv files whose charset was not UTF-8, say latin1 or cp1252.  As it was, we would obtain an irreversibly damaged representation of the csv file that included lots of UTF-8 replacement characters, U+FFFD.

Thanks!