Escape multibyte line terminators in JSON encoding

Currently, json/encoding respects the JSON spec (as it should) which 
disallows \n and \r inside strings, escaping them as expected.

Unfortunately, ECMA-262 (Javascript) disallows not only \n and \r in 
strings, but "Line Terminators" which includes U+2028 and U+2029. 
See here: http://bclary.com/2004/11/07/#a-7.3

This pull request adds U+2028 and U+2029 to be escaped.

# Why? 

It's very common to see something like this in a Rails template:

<script type="text/javascript"> 
var posts = <%= @posts.to_json %>;
</script>

If U+2028 or U+2029 are part of any attributes output in the to_json
call, you will end up with an exception.
In Chrome: Uncaught SyntaxError: Unexpected token ILLEGAL 

# Why not?

This is JSON encoding, and the JSON spec is specific about how to 
encode strings. U+2028 and U+2029 don't get special treatment.

Just trying to start a discussion... what do you do in your apps
to deal with this? Is there a convention I'm missing?
Added escaping of U+2028 and U+2029 inside the json encoder.

U+2028 and U+2029 are allowed inside strings in JSON (as all literal
Unicode characters) but JavaScript defines them as newline
seperators. Because no literal newlines are allowed in a string, this
causes a ParseError in the browser. We work around this issue by
replacing them with the escaped version. The resulting JSON is still
valid and can be parsed in the browser.

This commit has been coauthored with Viktor Kelemen @yikulju
Merge pull request #10057 from zackham/patch-1

Escape multibyte line terminators in JSON encoding
Fix syntax error on Ruby 2.0

Since Ruby 2.0 is UTF-8 by default we need to explictly say that the
encoding of this file is US-ASCII
Merge pull request #10534 from cmaruz/master

Escape of U+2028 and U+2029 in the JSON Encoder

Conflicts:
	activesupport/lib/active_support/json/encoding.rb
Give credits to all the envolved people [ci skip]
Merge branch 'fix-json-encoding'

This is the compination of #10057 and 10534.

Closes #10320