Fixed bug in that user specified host/port/proxy were being overwritten

Now the environment variables HOST, PORT and DASH_PROXY are fallbacks rather than overwriting the user specified arguments for host, port, proxy

Author: trbedwards

dash/dash.py

@@ -2137,9 +2137,9 @@ def run(
 
         # Evaluate the env variables at runtime
 
-        host = os.getenv("HOST", host)
-        port = os.getenv("PORT", port)
-        proxy = os.getenv("DASH_PROXY", proxy)
+        host = host or os.getenv("HOST")
+        port = port or os.getenv("PORT")
+        proxy = proxy or os.getenv("DASH_PROXY")
 
         # Verify port value
         try: