No need to add config for x_sendfile_header

Rack::Sendfile is loaded by default now
Added CheckPending middleware in default

As this middleware comes by default in a new
rails app

Added test to check omit for CheckPending when

Active Record is not included.
Merge pull request #11448 from arunagw/updated_middleware_test

No need to add config for x_sendfile_header
Remove deprecation warning from attribute_missing 

for attributes that are columns.
Merge pull request #11282 from arunagw/deprecation-removed-attribute-missing-activerecord

Remove deprecated `attribute_missing`.
Do not re-save destroyed association on saving parent object

Closes #11450
Merge pull request #11451 from jetthoughts/11450_do_not_resave_destroyed_association

Do not re-save destroyed association on saving parent object
use arel rather than slapping together SQL strings
reorder bind parameters when merging relations
removing useless assingment
save another array allocation