[seiga] fix file extension and xml parsing

- The file extension of the first image had been used for all further
  images
- API responses can contain invalid characters, which cause the XML
  parser to fail (http://seiga.nicovideo.jp/user/illust/26377934
  contains several \x08 characters)

gallery_dl/text.py

@@ -15,6 +15,20 @@ import html
 import urllib.parse
 
 
+INVALID_XML_CHARS = [
+    chr(c) for c in (1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 14, 15, 16, 17, 18,
+                     19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31)
+]
+
+
+def clean_xml(xmldata, repl=""):
+    """Replace/Remove invalid control characters in XML data"""
+    for char in INVALID_XML_CHARS:
+        if char in xmldata:
+            xmldata = xmldata.replace(char, repl)
+    return xmldata
+
+
 def remove_html(text):
     """Remove html-tags from a string"""
     return " ".join(re.sub("<[^>]+?>", " ", text).split())