mysql can't handle a parallel thread pinging the connection, so we can
get wrong results or segvs

This reverts commit 7cc588b6.

Removed unnecessary block

Closes #9824.

Fix some typos in AR- CHANGELOG, tests, method doc. fixed

Fix a typo

Changed the call to .sort.last to .max when computing the migration version

Commas aren't comas.

Type a fixo.

do not actually need to sort everything (incurs more cost than just a
simple max).

Fix improperly configured host in generated urls

Fix guides error on error class name

The error that's raised in case the required key is missing it's actually `ActionController::ParameterMissing` not `ActionController::MissingParameter`. It's important to fix so that people reading the guides knows what error to rescue from when needed.

Find() on an association with an inverse will now scan existing in-memory records

remove unused payloads in blocks in caching instrumentation

if the association already holds that record in memory before checking
the database for the specified ids.

fix sentence [ci skip]

If the host in `default_url_options` is accidentally set with a protocol such as 

```
host: "http://example.com"
``` 

then the generated url will have the protocol twice `http://http://example.com` which is not what the user intended. Likely they wanted to define a host `host: "example.com"` and a `protocol: "http://"` but did not know the convention.

This may not the most common problem, but when it happens it can go undetected for a while. I accidentally added `http://` out of habit recently only to find all the links in my emails were broken after deploying a demo site to production. Rather than allow this accident go undetected, we can fix the problem in line by properly setting the protocol and host.


I was able to find this related question on stack overflow: http://stackoverflow.com/questions/5878329/rails-3-devise-how-do-i-make-the-email-confirmation-links-use-secure-https-n where the answer was highly upvoted.

This is based off of work in #7415 cc/ @pixeltrix

ATP Action Mailer and Action Pack

Fixed the repair_validations helper method.

it only cleared the validations that created :validate callbacks. This
didn't include the validates created by validates_with, so I've added a
method to clear all validations.

Fix warning: mismatched indentatmmismatched indentations at 'end' with 'def'

drop an unused hash; change slang to SPECIAL

Check 862389c9 for more background.

Check pull request #9789 for more information.

This reverts commit 9c4c05fc, reversing
changes made to 4620bdce.

Reason: They're not completely interchangeable, since blank? will also
check for strings containing spaces.

Change from blank? to empty? on obvious string values  to save extra calls

Introduced in 5dc2e353.

This reverts commit c24528fb.

Change from each to each_value;drop assignment in habtm

2. drop assignment of value to sum in test