fix: use C locale by default

[SGSSGene]

fix #1282

This explicitly sets locale 'C' to all std::stringstream objects that output YAML.

[jbeder]

In terms of verification: the issue #1282 talks about "running under" LC_ALL=en_US.UTF-8 and similar. How does the test you wrote here relate to that? Is that an environment variable? (Can that be set/unset in the test?)

[SGSSGene]

In terms of verification: the issue #1282 talks about "running under" LC_ALL=en_US.UTF-8 and similar. How does the test you wrote here relate to that? Is that an environment variable? (Can that be set/unset in the test?)

So, by default the locale() is set to "" at startup, meaning LC_ALL has no influence. But in there case someone is setting std::locale::global(std::locale("")) which changes the global setting to follow LC_ALL/LC_NUMERIC/etc. Instead of setting it to std::locale("") which reads from LC_ALL we can set it to a value of our wishes.

A locale set to en_US.UTF-8 would print 2014 as 2,014
Setting the same to de_DE.UTF-8 would print 2014" as 2.014`. Both things we really do not want.

So instead I overwrite locally for every std::stringstream it explicitly to "C", which I believe is the behavior we want.

Edit:
This test will fail (by throwing an exception) if en_US.UTF-8 is not available.