Add an option to use query string as is for validation

When validating request or response signature in process_slo() we
currently rebuild query string from 'get_data' elements. This requires
URL encoding components of the string. Unfortunately, some IdPs (AD,
ADFS) use lower-case encoding. To handle this, one needs to pass
lowercase_urlencoding=True. This complicates code that needs to support
different IdPs.

Instead, if 'query_string' is passed, use it as-as.

src/onelogin/saml2/auth.py

@@ -660,16 +660,14 @@ def __validate_signature(self, data, saml_type, raise_exceptions=False):
             if isinstance(sign_alg, bytes):
                 sign_alg = sign_alg.decode('utf8')
 
-            lowercase_urlencoding = False
-            if 'lowercase_urlencoding' in self.__request_data.keys():
-                lowercase_urlencoding = self.__request_data['lowercase_urlencoding']
-
-            signed_query = self.__build_sign_query(data[saml_type],
-                                                   data.get('RelayState', None),
-                                                   sign_alg,
-                                                   saml_type,
-                                                   lowercase_urlencoding
-                                                   )
+            signed_query = self.__request_data.get('query_string')
+            if not signed_query:
+                lowercase_urlencoding = self.__request_data.get('lowercase_urlencoding', False)
+                signed_query = self.__build_sign_query(data[saml_type],
+                                                       data.get('RelayState'),
+                                                       sign_alg,
+                                                       saml_type,
+                                                       lowercase_urlencoding)
 
             if exists_multix509sign:
                 for cert in idp_data['x509certMulti']['signing']: