replace '&' with & when it is not in a character entity (approximation)

pylast.py

@@ -33,6 +33,7 @@ import tempfile
 import sys
 import collections
 import warnings
+import re
 
 def _deprecation_warning(message):
     warnings.warn(message, DeprecationWarning)
@@ -847,6 +848,10 @@ class _Request(object):
         except Exception as e:
             raise MalformedResponseError(self.network, e)
 
+        # Pretty decent catch for invalid & characters - which last.fm
+        # seems to generate for some artist eg. "K'nann"
+        response_text = re.sub("&(?![^\W]+;)", "&", response_text)
+
         self._check_response_for_errors(response_text)
         return response_text