Remove leading whitespace from the XML under test

This test was originally written with the intention of asserting that
a runtime error related to XXE will be raised by the parser. However,
because initial whitespace is present before the doctype,
XmlMini_NokogiriSAX::HashBuilder has been raising an unrelated error
in this test.

Related to #41015

---

Using Nokogiri v1.10.10, the error being raised without this change
is:

> XML declaration allowed only at the start of the document

but with this change we see the expected exception from the libxml2
SAX parser:

> Entity 'a' not defined

Using Nokogiri v1.11.0, in which error handling is broken (see
sparklemotion/nokogiri#2168), without this change we see an exception
being raised by HashBuilder because `characters` is called before
`start_element` and so the content hash isn't initialized (see

The error being raised with this change is:

> Parse stack not empty!

which is not the error we want (because of
sparklemotion/nokogiri#2168), but the test still passes.

Using Nokogiri with the fix from sparklemotion/nokogiri#2169, the
error being raised without this change is:

> XML declaration allowed only at the start of the document

but with this change will be:

> Entity 'a' not defined

and we're back to the expected behavior.

activesupport/test/xml_mini/xml_mini_engine_test.rb

@@ -50,7 +50,7 @@ class XMLMiniEngineTest < ActiveSupport::TestCase
 
     def test_exception_thrown_on_expansion_attack
       assert_raise expansion_attack_error do
-        Hash.from_xml(<<-eoxml)
+        Hash.from_xml(<<~eoxml)
           <?xml version="1.0" encoding="UTF-8"?>
           <!DOCTYPE member [
             <!ENTITY a "&b;&b;&b;&b;&b;&b;&b;&b;&b;&b;">