ConnectionPool, unify exceptions, ConnectionTimeoutError

Use mysqldump native commands when rake db:structure:dump. Closes #5547
Fixes database tasks config to use encoding instead of charset.

As a result of different commits, ConnectionPool had become
of two minds about exceptions, sometimes using PoolFullError
and sometimes using ConnectionTimeoutError. In fact, it was
using ConnectionTimeoutError internally, but then recueing
and re-raising as a PoolFullError.

There's no reason for this bifurcation, standardize on
ConnectionTimeoutError, which is the rails2 name and still
accurately describes semantics at this point.

History

In Rails2, ConnectionPool raises a ConnectionTimeoutError if
it can't get a connection within timeout.

Originally in master/rails3, @tenderlove had planned on removing
wait/blocking in connectionpool entirely, at that point he changed
exception to PoolFullError.

But then later wait/blocking came back, but exception remained
PoolFullError.

Then in 02b23355 pmahoney introduced fair waiting logic, and
brought back ConnectionTimeoutError, introducing the weird bifurcation.

ConnectionTimeoutError accurately describes semantics as of this
point, and is backwards compat with rails2, there's no reason
for PoolFullError to be introduced, and no reason for two
different exception types to be used internally, no reason
to rescue one and re-raise as another.  Unify!

Have rails dbconsole pass sslca to the mysql command line client.

Improve exception message for HasManyThroughAssociationPolymorphicSourceError

Add .rb template handler

This handler simply allows arbitrary Ruby code as a template

Rake assets:precompile shouldn't fail quietly.

We should show to user original issue if rake task will be aborted.

Revert "Changelog entry for PR #7560"

This reverts commit ca80067b.
Entry already added in 895233

Changelog entry for PR #7560

Suppress  annoy warning, when executing AR's testcases.

Exception message was misleading, as it is possible to have a
polymorphic 'has_many :through' join model.

ERB::Util.html_escape encodes single quote as #39. Decimal form has better support in old browsers.

Conflicts:
	activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

Use internal instance variable naming scheme for mounted URL helpers

removed warning: shadowing outer local variable

Raise MissingAttributeError on query methods

When calling a query method on an attribute that was not selected by
an ActiveRecord query, an ActiveModel::MissingAttributeError is not
raised. Instead, a nil value is returned, which will return false once
cast to boolean.

This is undesirable, as we should not give the impression that we know
the attribute's boolean value when we haven't loaded the attribute's
(possibly) non-boolean value from the database.

This issue is present on versions going back as far as 2.3, at least.

set default_headers to nil after use it to avoid order dependent tests

Fixed missing require

Missing require caused fail of guide generation (in
action_dispatch/http/mime_type, line 295, undefined method `ends_with`
for "to_ary":String)
With this fix guides were normally generated

improve AR CHANGELOG [ci skip]