Fix "http" service plugin on Python 3

If "srv.SCRIPTNAME" was None, the plugin croaked on Python 3 with
"expected string or bytes-like object".

Author: amotl

CHANGES.rst

@@ -7,6 +7,7 @@ in progress
 ===========
 
 - Rename repository default branch to "main"
+- Fix "http" service plugin
 
 2021-06-12 0.24.0
 =================

codecov.yml

@@ -5,5 +5,5 @@ coverage:
   status:
     project:
       default:
-        #target: 85%    # the required coverage value
+        target: 14%    # the required coverage value
         threshold: 3%  # the leniency in hitting the target

mqttwarn/core.py

@@ -708,7 +708,8 @@ def bootstrap(config=None, scriptname=None):
     invoker = FunctionInvoker(config=config, srv=make_service(mqttc=None, name='mqttwarn.context'))
     context = RuntimeContext(config=config, invoker=invoker)
     cf = config
-    SCRIPTNAME = scriptname
+    if scriptname is not None:
+        SCRIPTNAME = scriptname
 
 
 def run_plugin(config=None, name=None, data=None):

mqttwarn/services/http_urllib.py

@@ -75,8 +75,9 @@ def plugin(srv, item):
                 resource = url
 
             request = urllib.request.Request(resource)
-            request.add_header('User-agent', srv.SCRIPTNAME)
 
+            if srv.SCRIPTNAME is not None:
+                request.add_header('User-agent', srv.SCRIPTNAME)
             if auth is not None:
                 request.add_header("Authorization", "Basic %s" % auth)
 
@@ -107,9 +108,12 @@ def plugin(srv, item):
 
 
             request.data = encoded_params.encode('utf-8')
-            request.add_header('User-agent', srv.SCRIPTNAME)
+
+            if srv.SCRIPTNAME is not None:
+                request.add_header('User-agent', srv.SCRIPTNAME)
             if auth is not None:
                 request.add_header("Authorization", "Basic %s" % auth)
+
             srv.logging.debug("before send")
             resp = urllib.request.urlopen(request, timeout=timeout)
             data = resp.read()