Address review feedback: optional sip_uri, set_sip_uri setter

Make sip_uri optional (default None) to match how other verbs handle
their nested verbs, guard nested_verbs construction against None, and
add a set_sip_uri() setter so callers can attach the SipUri after
construction (mirrors the ruby/node Refer pattern). Replaced
test_minimal/test_shares_sip_uri_with_transfer with a test covering
the new setter.

Author: stampercasey

bandwidth/models/bxml/verbs/refer.py

@@ -12,7 +12,7 @@
 class Refer(NestableVerb):
 
     def __init__(
-        self, sip_uri: SipUri,
+        self, sip_uri: SipUri=None,
         refer_complete_url: str=None, refer_complete_method: str=None,
         tag: str=None
     ):
@@ -47,7 +47,7 @@ def __init__(
         self.tag = tag
         super().__init__(
             tag="Refer",
-            nested_verbs=[self.sip_uri]
+            nested_verbs=[sip_uri] if sip_uri is not None else []
         )
 
     @property
@@ -57,3 +57,12 @@ def _attributes(self):
             "referCompleteMethod": self.refer_complete_method,
             "tag": self.tag
         }
+
+    def set_sip_uri(self, sip_uri: SipUri) -> None:
+        """Set the SIP URI destination for this <Refer> verb.
+
+        Args:
+            sip_uri (SipUri): The SIP URI to refer the call to.
+        """
+        self.sip_uri = sip_uri
+        self._nested_verbs = [sip_uri]

test/unit/models/bxml/test_refer.py

@@ -30,15 +30,8 @@ def test_to_bxml(self):
         expected = '<Refer referCompleteUrl="https://example.com/handleRefer" referCompleteMethod="POST" tag="test"><SipUri>sip:alice@atlanta.example.com</SipUri></Refer>'
         assert expected == self.refer.to_bxml()
 
-    def test_minimal(self):
-        minimal_refer = Refer(sip_uri=SipUri(uri="sip:bob@example.com"))
+    def test_set_sip_uri(self):
+        refer = Refer()
+        refer.set_sip_uri(SipUri(uri="sip:bob@example.com"))
         expected = '<Refer><SipUri>sip:bob@example.com</SipUri></Refer>'
-        assert expected == minimal_refer.to_bxml()
-
-    def test_shares_sip_uri_with_transfer(self):
-        """Refer uses the same SipUri model as Transfer, including its
-        transfer-flavored attributes if the caller sets them."""
-        sip_uri = SipUri(uri="sip:alice@atlanta.example.com", uui="test")
-        refer = Refer(sip_uri=sip_uri)
-        expected = '<Refer><SipUri uui="test">sip:alice@atlanta.example.com</SipUri></Refer>'
         assert expected == refer.to_bxml()