Make UTF-8 the default encoding for XML feeds

Consider the feed http://planet.haskell.org/atom.xml
- This is a UTF-8 encoded XML file
- No encoding declaration in the XML header
- No Unicode byte order mark
- Served with HTTP Content-Type "text/xml" (no charset parameter)

Miniflux lets charset.NewReader handle this. The charset package
implements the HTML5 character encoding algorithm, which, in this
situation, defaults to windows-1252 encoding if there are no UTF-8
characters in the first 1000 bytes. So for this feed, we get the wrong
encoding.

I inserted an explicit "utf8.Valid()" check, which fixes this problem.

http/client/response.go

@@ -11,6 +11,7 @@ import (
 	"mime"
 	"regexp"
 	"strings"
+	"unicode/utf8"
 
 	"golang.org/x/net/html/charset"
 )
@@ -97,6 +98,12 @@ func (r *Response) EnsureUnicodeBody() (err error) {
 			if xmlEncodingRegex.Match(buffer[0:length]) {
 				return
 			}
+
+			// If no encoding is specified in the XML prolog and
+			// the document is valid UTF-8, nothing needs to be done.
+			if utf8.Valid(buffer) {
+				return
+			}
 		}
 	}