Decimal separator follows the locale

[miloyip]

Upstream https://code.google.com/p/rapidjson/issues/detail?id=84

A locale may output "3,1416" instead of "3.1416" due to locale setting. JSON should always uses . as decimal separator. Need to also check other similar issues such as digit grouping, e.g. "123,456.789" is invalid JSON number.

[miloyip]

Currently Writer::Double() simply uses snprintf() to convert a double to string.
snprintf() uses the locale settings set by setlocale().
An idea is to setlocale(LC_NUMERIC, "C") before snprintf() and restore the locale after. However, there is no getlocale() in standard C library for backup.
Another way is to develop a custom dtoa() which neglects locale altogether. This way the code can be potentially optimized .

Some implementations can be referred to:

1. netlib's dtoa
2. double-conversion
3. stringencoders

However, floating-point to string conversion is much complicated than integer, e.g. [1][2].

[1] Burger, Robert G., and R. Kent Dybvig. "Printing floating-point numbers quickly and accurately". ACM SIGPLAN Notices. Vol. 31. No. 5. ACM, 1996.)
[2] Kupriianova, Olga, Christoph Lauter, and Jean-Michel Muller. "Radix conversion for IEEE754-2008 mixed radix floating-point arithmetic". Signals, Systems and Computers, 2013 Asilomar Conference on. IEEE, 2013.

[pah]

You can use setlocale for querying the locale as well:

If locale is a null pointer, setlocale queries the current C locale without modifying it.

On the other hand, it may be quite costly to fiddle with the locale in each call to Writer::Double. As a first step, it could be added to the documentation that the user should take care for this in the application code.