Fix decoded_content

`Furl::Response::decoded_content` returns `undef` when specified `Accept-Encoding: gzip`.

```perl
#!perl
use strict;
use warnings;
use Furl;

my $ua = Furl->new( headers => [ 'Accept-Encoding' => 'gzip' ]);

my $res = $ua->get('http://64p.org');
die "Can't download" unless $res->is_success;
print $res->decoded_content;
```

I got following output
```
Use of uninitialized value in print at furl.pl line 10.
```

I think HTTP::Message::decoded_content fails because it takes header `Content-Encoding: gzip`
but it takes inflated content. So I fix that `decoded_content` removes header value about
data compression.

Please see this patch.