add unit test and documentation for custom HTTP headers (#285)

examples/basic_usage_examples.py

@@ -18,6 +18,7 @@
 caldav_url = "https://calendar.example.com/dav"
 username = "somebody"
 password = "hunter2"
+headers = {"X-MY-CUSTOMER-HEADER" : "123"}
 
 
 def run_examples():
@@ -30,7 +31,10 @@ def run_examples():
     ## so the credentials aren't validated.
     ## The client object can be used as a context manager, like this:
     with caldav.DAVClient(
-        url=caldav_url, username=username, password=password
+        url=caldav_url,
+        username=username, 
+        password=password,
+        headers = headers # Optional parameter to set HTTP headers on each request if needed
     ) as client:
 
         ## Typically the next step is to fetch a principal object.

tests/test_caldav_unit.py

@@ -264,6 +264,18 @@ def testRequestNonAscii(self, mocked):
         assert response.status == 200
         assert response.tree is None
 
+    @mock.patch("caldav.davclient.requests.Session.request")
+    def testRequestCustomHeaders(self, mocked):
+        """
+        ref https://github.com/python-caldav/caldav/issues/285
+        """
+        mocked().status_code = 200
+        mocked().headers = {}
+        cal_url = "http://me:hunter2@calendar.møøh.example:80/"
+        client = DAVClient(url=cal_url, headers={"X-NC-CalDAV-Webcal-Caching": "On"})
+        assert client.headers["Content-Type"] == "text/xml"
+        assert client.headers["X-NC-CalDAV-Webcal-Caching"] == "On"
+
     @mock.patch("caldav.davclient.requests.Session.request")
     def testEmptyXMLNoContentLength(self, mocked):
         """