Add language directive to .travis.yml

the ERB has already been compiled to Ruby code by the time we're calling
module_eval.  Nothing that module eval raises will be caught by a blank
`rescue`, so I think we can remove this

Currently, Travis CI assumes that the project is in Ruby in the
absence of the `language` key.
This behavior may change in the future. (For example, switch to a "blank"
image which *may* be created.)

Refactor out Dir.glob from ActionDispatch::Static

Dir.glob can be a security concern. The original use was to provide logic of fallback files. Example a request to `/` should render the file from `/public/index.html`. We can replace the dir glob with the specific logic it represents. The glob {,index,index.html} will look for the current path, then in the directory of the path with index file and then in the directory of the path with index.html. This PR replaces the glob logic by manually checking each potential match. Best case scenario this results in one less file API request, worst case, this has one more file API request.

Related to #16464

Update: added a test for when a file of a given name (`public/bar.html` and a directory `public/bar` both exist in the same root directory. Changed logic to accommodate this scenario.

[ci skip] mention assert_select in release notes

Use less iterations for KeyGenerator in tests

update 4_2_release_notes

Correctly detect mutation on serialized columns mapping to binary

[ci skip] Fix typo of `collection_singular_ids=` method.

[ci skip] Fix typo of `collection=` method.

Fixes #16701

Fix spelling. [CI SKIP]

This was caused by #15327.

Update documentation to match change in #5942 [ci skip]

This commit improves performance of cookie tests:

Ruby  |   After  |   Before
----- | --------:| --------:
MRI   |    5.03s |     9.28s
JRuby |   25.45s |  1648.23s

Please note the improvement for JRuby.

[ci skip] Modify a web-console PR link for release noted

[ci skip] Updated documentation syntax of other parameter for rdoc

Adjusted punctuation and grammar for Form Helpers guide [ci skip]

[ci skip] Updated documentation syntax of block parameter for rdoc

[ci skip] Updated include to reference record parameter for rdoc

[ci skip] Updated documentation syntax of block parameter for rdoc

[ActiveJob] Autoload adapters

Speed up JRuby tests by improving the startup time

Otherwise, it's possible for GC to run in between, and fail the test.