Ruby: Fix bug in excon model

If a codebase included a definition for `Excon.new`, we matched
connection nodes to unrelated request nodes.

ruby/ql/test/query-tests/security/cwe-295/Excon.rb

@@ -46,4 +46,23 @@ def method8
   # GOOD
   Excon.defaults[:ssl_verify_peer] = false
   Excon.new("http://example.com/", ssl_verify_peer: true)
+end
+
+# Regression test for excon
+
+class Excon
+  def self.new(params)
+    Excon::Connection.new(params)
+  end
+end
+
+def method9
+  # GOOD: connection is not used
+  Excon.new("foo", ssl_verify_peer: false)
+end
+
+def method10
+  # GOOD
+  connection = Excon.new("foo")
+  connection.get("bar")
 end