[#432][#403][#433][#422] Fix checking for column cache until db connects

[bf4]

Fix related to #432, #403, #433, #422

Summary: When ActsAsTaggableOn::Taggable::Cache is included, it conditionally adds tag caching methods when the model has any "cached_#{tag_type}_list" column

Details: ActiveRecord::Base.columns makes a database connection and caches the calculated
columns hash for the record as @columns. Since we don't want to add caching
methods until we confirm the presence of a caching column, and we don't
want to force opening a database connection when the class is loaded,
here we intercept and cache the call to :columns as @acts_as_taggable_on_columns
to mimic the underlying behavior. While processing this first call to columns,
we do the caching column check and dynamically add the class and instance methods

Also see
#432 (comment)

Very simplified description of error condition in console

# pre-PR behavior, methods loaded iff db connection exists when model loaded
# User loaded before db connection
ActiveRecord::Base.connected? 
=> false
 User.caching_tag_list_on?
=> NoMethodError: undefined method `caching_tag_list_on?' for User(no database connection):Class
ActiveRecord::Base.connection
=>  ActiveRecord::ConnectionAdapters::SQLite3Adapter 
 User.caching_tag_list_on?
=> NoMethodError: undefined method `caching_tag_list_on?' for User(no database connection):Class

# User loaded after db connection
ActiveRecord::Base.connection
=>  ActiveRecord::ConnectionAdapters::SQLite3Adapter 
 User.caching_tag_list_on?
=> ArgumentError: wrong number of arguments (0 for 1)

# post-PR behavior, methods loaded on db connection
ActiveRecord::Base.connected? 
 => false 
 User.caching_tag_list_on?
NoMethodError: undefined method `caching_tag_list_on?' for User(no database connection):Class
ActiveRecord::Base.connection
=>  ActiveRecord::ConnectionAdapters::SQLite3Adapter 
 User.caching_tag_list_on?
ArgumentError: wrong number of arguments (0 for 1)

[bf4]

@seuros @aaronchi @shekibobo @joelcogen @dstosik can you confirm this PR fixes the issue for you?

[seuros]

It seem the the issue is resolved. 👍 Thank you!

[bf4]

Hmm, looks like we're calling this method twice.. but that's existing behavior

[bf4]

@seuros Thanks, since I was only able to duplicate it console, and need to think about how to test.

[bf4]

The check for table_exists? should not be necessary here