Fix regex to detect base64 encoded messages

Author: pitbulk

lib/onelogin/ruby-saml/saml_message.rb

@@ -19,8 +19,7 @@ class SamlMessage
       ASSERTION = "urn:oasis:names:tc:SAML:2.0:assertion"
       PROTOCOL  = "urn:oasis:names:tc:SAML:2.0:protocol"
 
-      BASE64_FORMAT = %r(\A[A-Za-z0-9+/]{4}*[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=?\Z)
-
+      BASE64_FORMAT = %r(\A([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?\Z)
       @@mutex = Mutex.new
 
       # @return [Nokogiri::XML::Schema] Gets the schema object of the SAML 2.0 Protocol schema