`silence_stderr`, `silence_stream`, `capture` and `quietly`.

`MissingSourceFile` was just an alias to `LoadError` and was not
being raised inside the framework.

They are already present on Ruby 2.2

This constant may be define for auxiliar gems like rails-html-sanitizer
and these methods call will fail.

When trying to access a character on a string buffer object via `:[]`, if the object being accessed currently returns `html_safe?` as true,
we used to set  `@html_safe` variable as true on new object created. When doing something like

x = 'Hello'.html_safe
x[/a/, 1]

would throw an error on ruby 2.2, since when nothign gets matched nil is returned by the code and it tries to set   `@html_safe` value to true,
which would error since starting 2.2 nil is frozen.

This change adds a safety net to avoid setting `@html_safe = true` on frozen objects.

Fixes #18235

Add examples for missing_name, missing_name?

[ci skip]

Also add doc examples for `Object.nil`.

[ci skip]

Add docs for `minutes`, `hours`, `days`, `weeks` and `fortnights`.
Fix docs for `in_milliseconds`.

[ci skip]

@rafaelfranca suggested in f7c7bcd9 that code examples should display
the result after  `# =>` and not after `#=>`.

This commit replaces *all* the occurrences of `#=>` in the code documentation
(mostly added by me 😭) with the suggested `# =>`.

Add docs for `kilobytes`, `megabytes`, `gigabytes`, `terabytes`,
`petabytes` and `exabytes`. Fix docs for `bytes`.

[ci skip]

Also improves docs for `Time#find_zone!`

[ci skip]

Adds examples and keeps coherent with the documentation of the
similar method `seconds_until_end_of_day`. [ci skip]

It should be part of the documented public API, since we have an entire
section of the guides dedicated to it. Documented in a way that
addresses the concerns which kept it undocumented in the past.

pattern removal

added example for string#remove and test case for remove of multiple occurence of pattern

removed extra whitespaces

This patch uniformizes warning messages. I used the most common style
already present in the code base:

* Capitalize the first word.

* End the message with a full stop.

* "Rails 5" instead of "Rails 5.0".

* Backticks for method names and inline code.

Also, converted a few long strings into the new heredoc convention.

@carlosantoniodasilva pointed out that when `@person` is nil then this would blow up when you ended up calling `#first`on `nil`.

> "there’s no way to break a try chain when you enter it :D"

[ci skip]

- better `if` example
- Added chaining example to the try method description
- Documented the `respond_to?` check to the try method description
- Clearer wording to explain that argument error is raised on argument mismatch to responding method, rather than to non-responding method (which is handled without exception by `try`)
- `.any?` is more precise than `! .blank?`
 - Don't need to use `try` on `children` as (for regular associations) they will always be a collection or array that responds to `first`
- Fix typos/grammar

Based on commit 5e51bdda.

[ci skip] Add Doc of with_options for the case when inherited default options and original options have same keys

atomic_write rescue also Errno::EACCES on changing file permission. It could be raised with some type of filesystem