Cleanup of excerpt helper

* replaced String concatenation by joining
* separator has default value to '', even it is nil
ask the association for records rather than calling `send`
add a comment for sanity of other people to come
Add meta tag with charset information to application layout.

Previously, our default HTML would validate properly, but would generate
a warning: it doesn't declare a character encoding.

According to [the spec][encoding-spec], if you don't specify an
encoding, a 7 step algorithm happens, with a toooon of sub-steps. Or, we
could just actually specify it.

Since everything else in Rails assumes UTF-8, we should make sure pages
are served with that encoding too. This meta tag is the simplest way to
accomplish this.

More resources:

* http://blog.whatwg.org/the-road-to-html-5-character-encoding
* http://www.w3.org/International/tutorials/tutorial-char-enc/
* http://validator.w3.org/

[encoding-spec]: http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html#determining-the-character-encoding
[ci skip] 'previous version of Rails' is gramatically incorrect
Perf: avoid dupes add fallback logic for coders
Merge pull request #12184 from waynn/patch-4

"previous version of Rails" is gramatically incorrect
Reduce allocations when extracting AR models
Relation#merge should not lose readonly(false) flag.

The original code ignores the `false` value because `false.blank? # => true`.
Reduce Duration#inspect to a single series of transformations

* eliminates need for temp Hash

Also added a couple of examples to DurationTest to specify:

* duration can be defined with units out of order e.g.
  1.month + 1.year + 1.second + 1.day
* equality with a Fixnum works regardless of which operand is on which
  side of the operator
remove sentence err
Merge pull request #12199 from michaelminter/master

remove sentence err
fix deleting join models with no pk
Avoid empty transaction from setting has_one association on new record.
Revert "Add meta tag with charset information to application layout."

This reverts commit ba0407337e93c4ef55cef3472143f62e8a984a64.
Merge pull request #12194 from thedarkone/readonly-merger-fix

Relation#merge should not lose readonly(false) flag.
Merge pull request #12185 from SamSaffron/join_dep

Reduce allocations when extracting AR models
Whitespaces
Merge pull request #12135 from dylanahsmith/avoid_empty_transaction

Avoid empty transaction from setting has_one association on new record.

Conflicts:
	activerecord/CHANGELOG.md
Merge pull request #12200 from dchelimsky/simplify-duration-inspect-even-more

Reduce Duration#inspect to a single series of transformations