only enforce host presence for HTTP / HTTPS

Requests supports adding other protocols by mounting an adapter for them.  For
protocols that don't require a host entry, this handling is overzealous, and
prevents useful URLs from making it through.  This change only enforces this
for the http and https protocol schemes, and adjusts the rest of the URL
handling to be more friendly to non-present network locations.

requests/models.py

@@ -327,20 +327,22 @@ def prepare_url(self, url, params):
             raise MissingSchema("Invalid URL {0!r}: No schema supplied. "
                                 "Perhaps you meant http://{0}?".format(url))
 
-        if not host:
+        if scheme in ('http', 'https') and not host:
             raise InvalidURL("Invalid URL %r: No host supplied" % url)
 
         # Only want to apply IDNA to the hostname
         try:
-            host = host.encode('idna').decode('utf-8')
+            if host:
+                host = host.encode('idna').decode('utf-8')
         except UnicodeError:
             raise InvalidURL('URL has an invalid label.')
 
         # Carefully reconstruct the network location
         netloc = auth or ''
         if netloc:
             netloc += '@'
-        netloc += host
+        if host:
+            netloc += host
         if port:
             netloc += ':' + str(port)