docs, AR already auto-detects primary keys. Closes #13946. [ci skip]

This behavior was introduced since Rails 3.1 (207f266ccaaa9cd04cd2a7513ae5598c4358b510)
but the docs were still out of date.
Fixed typo in column_definition_test.rb.
Merge pull request #13949 from vajrasky/fix_typo_column_definition_test

Fixed typos in column_definition_test.rb.
Fixed typos [ci skip]
Merge pull request #13950 from calvintam/fix_typos

Fixed typos
Removed unused commented code [ci skip]

As discussed in #13940 this can be removed
We can document this somewhere else in testing guides.
remove gemnasium batch from README. [ci skip]

The rails/rails gemnasium page (https://gemnasium.com/rails/rails) is not helping much.
It lists 0 dependencies so there is not much to track. Also our gems mostly depend
on other gems we control. There is not much point in monitoring them.

The batch was introduced with #3909.
Remove obsolete line (was needed for Ruby 1.8.7 support)
Remove obsolete test (builtin group_by is now used)
Strengthen test with different nb of rows and columns
Return sized enumerator from Batches#find_in_batches
Return sized enumerator from Batches#find_each
Return sized enumerator from Enumerable#index_by
Merge pull request #13938 from marcandre/sized_enumerator

Sized enumerator
fix interplay of humanize and html_escape
Merge pull request #13953 from seanwalbran/fix-humanize-match-reference

fix interplay of humanize and html_escape
Fixed "Hash#to_param confused by empty hash values #13892"
Move test to the right file
Test with a blank value
Refatoring the method to avoid shot-circuit return