Ignore certain MIME types in fixCharset

Some MIME types do not involve any text data and should not be processed
in fixCharset.  This should be done instead of forcing the user to
toggle the Collector.DetectCharset flag as it could prove problematic.
Fix gocolly#312

response.go

@@ -68,6 +68,16 @@ func (r *Response) fixCharset(detectCharset bool, defaultEncoding string) error
 		return nil
 	}
 	contentType := strings.ToLower(r.Headers.Get("Content-Type"))
+
+	if strings.Contains(contentType, "image/") ||
+		strings.Contains(contentType, "video/") ||
+		strings.Contains(contentType, "audio/") ||
+		strings.Contains(contentType, "font/") {
+		// These MIME types should not have textual data.
+
+		return nil
+	}
+
 	if !strings.Contains(contentType, "charset") {
 		if !detectCharset {
 			return nil