Split session JSON serialization and writing to file

To avoid writing invalid JSON in case of presence of unserializable data due to an internal bug.

httpie/config.py

@@ -108,16 +108,14 @@ class BaseConfigDict(dict):
 
         self.ensure_directory()
 
-        try:
-            with self.path.open('w') as f:
-                json.dump(
+        json_string = json.dumps(
             obj=self,
-                    fp=f,
             indent=4,
             sort_keys=True,
             ensure_ascii=True,
         )
-                f.write('\n')
+        try:
+            self.path.write_text(json_string + '\n')
         except IOError:
             if not fail_silently:
                 raise