Initialize @default_timezone and @timestamp_decoder in add_pg_decoders

Staled `@default_timezone` would cause an error on `reconnect!` after
`disconnect!`.

https://buildkite.com/rails/rails/builds/59495#23be8079-3a4f-4375-9991-0a6f874554f2

Steps to reproduce:

```
% ARCONN=postgresql bin/test test/cases/adapter_test.rb test/cases/base_test.rb -n "/(?:test_attributes_on_dummy_time|test_reconnect_after_a_disconnect)$/" --seed 15849
Using postgresql
Run options: -n "/(?:test_attributes_on_dummy_time|test_reconnect_after_a_disconnect)$/" --seed 15849

# Running:

.
E

Error:
ActiveRecord::AdapterTestWithoutTransaction#test_reconnect_after_a_disconnect:
NoMethodError: undefined method `add_coder' for #<PG::TypeMapAllStrings:0x00007f85ab9dd5b8>
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:866:in `update_typemap_for_default_timezone'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:652:in `exec_no_cache'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:636:in `execute_and_clear'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:894:in `add_pg_decoders'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:744:in `connect'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:285:in `rescue in block in reconnect!'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:281:in `block in reconnect!'
    /Users/kamipo/.rbenv/versions/2.6.1/lib/ruby/2.6.0/monitor.rb:230:in `mon_synchronize'
    /Users/kamipo/src/github.com/rails/rails/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb:280:in `reconnect!'
    /Users/kamipo/src/github.com/rails/rails/activerecord/test/cases/adapter_test.rb:465:in `block in <class:AdapterTestWithoutTransaction>'
```

Author: kamipo

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -247,8 +247,6 @@ def initialize(connection, logger, connection_parameters, config)
 
         # @local_tz is initialized as nil to avoid warnings when connect tries to use it
         @local_tz = nil
-        @default_timezone = nil
-        @timestamp_decoder = nil
         @max_identifier_length = nil
 
         configure_connection
@@ -869,6 +867,9 @@ def update_typemap_for_default_timezone
         end
 
         def add_pg_decoders
+          @default_timezone = nil
+          @timestamp_decoder = nil
+
           coders_by_name = {
             "int2" => PG::TextDecoder::Integer,
             "int4" => PG::TextDecoder::Integer,