Fix prettify if no content type is applied.

This can happen some time e.g some unauthorized responses do not have any content type

httpie/pretty.py

@@ -35,6 +35,9 @@ def prettify_http(headers):
 
 
 def prettify_body(content, content_type):
+    if not content_type:
+        return content
+
     content_type = content_type.split(';')[0]
     if 'json' in content_type:
         content_type = TYPE_JS