Refactor get_filename so it can be tested

Just pass in the headers instead of the whole response object so that it
can be tested without needing an actual server.

src/composer/http_client.py

@@ -26,6 +26,12 @@ from composer.unix_socket import UnixHTTPConnectionPool
 def api_url(api_version, url):
     """Return the versioned path to the API route
 
+    :param api_version: The version of the API to talk to. eg. "0"
+    :type api_version: str
+    :param url: The API route to talk to
+    :type url: str
+    :returns: The full url to use for the route and API version
+    :rtype: str
     """
     return os.path.normpath("/api/v%s/%s" % (api_version, url))
 
@@ -124,7 +130,7 @@ def post_url_json(socket_path, url, body):
                      headers={"Content-Type": "application/json"})
     return json.loads(r.data.decode("utf-8"))
 
-def get_filename(response):
+def get_filename(headers):
     """Get the filename from the response header
 
     :param response: The urllib3 response object
@@ -133,12 +139,12 @@ def get_filename(response):
     :returns: Filename from content-disposition header
     :rtype: str
     """
-    log.debug("Headers = %s", response.headers)
-    if "content-disposition" not in response.headers:
+    log.debug("Headers = %s", headers)
+    if "content-disposition" not in headers:
         raise RuntimeError("No Content-Disposition header; cannot get filename")
 
     try:
-        k, _, v = response.headers["content-disposition"].split(";")[1].strip().partition("=")
+        k, _, v = headers["content-disposition"].split(";")[1].strip().partition("=")
         if k != "filename":
             raise RuntimeError("No filename= found in content-disposition header")
     except RuntimeError:
@@ -163,7 +169,7 @@ def download_file(socket_path, url, progress=True):
         if not err["status"]:
             raise RuntimeError(err["error"]["msg"])
 
-    filename = get_filename(r)
+    filename = get_filename(r.headers)
     if os.path.exists(filename):
         msg = "%s exists, skipping download" % filename
         log.error(msg)