LogstashFormatter: Move field_skip_set creation to __init__

Commit baf2118 introduced a new set `field_skip_set` to store the fields to
skip, unfortunately this field is created too early, causing modification
of `FORMATTER_RECORD_FIELD_SKIP_LIST` by library users to be ignored.
Let's create the field at the end of __init__ instead to fix the regression.

Fixes #96

logstash_async/formatter.py

@@ -27,7 +27,6 @@ class LogstashFormatter(logging.Formatter):
 
     _basic_data_types = (type(None), bool, str, int, float)
 
-    field_skip_set = set(constants.FORMATTER_RECORD_FIELD_SKIP_LIST)
     top_level_field_set = set(constants.FORMATTER_LOGSTASH_MESSAGE_FIELD_LIST)
 
     class MessageSchema:
@@ -89,6 +88,8 @@ def __init__(
         self._prefetch_logsource()
         self._prefetch_program_name()
 
+        self.field_skip_set = set(constants.FORMATTER_RECORD_FIELD_SKIP_LIST)
+
     # ----------------------------------------------------------------------
     def _prefetch_interpreter(self):
         """Override when needed"""