Fix element signature for builtin AnyType

Fixes mvantellingen#1348

src/zeep/xsd/elements/element.py

@@ -309,7 +309,8 @@ def resolve(self):
     def signature(self, schema=None, standalone=True):
         from zeep.xsd import ComplexType
 
-        if self.type.is_global or (not standalone and self.is_global):
+        if ((self.type.is_global and self.type.qname is not None) or
+                (not standalone and self.is_global)):
             value = self.type.get_prefixed_name(schema)
         else:
             value = self.type.signature(schema, standalone=False)

tests/test_xsd_signatures.py

@@ -1,3 +1,4 @@
+import pytest
 from lxml import etree
 
 from tests.utils import load_xml
@@ -289,3 +290,14 @@ def test_schema_recursive_ref():
 
     elm = schema.get_element("ns0:Container")
     elm.signature(schema)
+
+
+@pytest.mark.parametrize('default_type', xsd.default_types.values())
+@pytest.mark.parametrize('standalone', [True, False])
+def test_signature_builtin_type(default_type, standalone):
+    builtin_type = xsd.Element(
+        etree.QName("http://tests.python-zeep.org/", "authentication"),
+        default_type
+    )
+    signature = builtin_type.signature(standalone=standalone)
+    assert isinstance(signature, str)