v3.0 - Fix REXML::Security.entity_expansion_limit global mutation

[johnnyshields]

RubySaml::XML::BaseDocument has a line:

REXML::Security.entity_expansion_limit = 0

This mutates the global state of REXML, and also means that RubySaml could be affected by other gems changing this.

Instead, we should do something like:

def with_secure_rexml
  old_eel = REXML::Security.entity_expansion_limit
  REXML::Security.entity_expansion_limit = 0
  yield
ensure
  REXML::Security.entity_expansion_limit = old_eel
end

It's not threadsafe however...

Maybe just replace REXML with Nokogiri?