Internationalized domain name (IDN) support

[alexeyshockov]

This PR introduces IDN support on client's level, so all the handlers will work with the ASCII version of the IDN with no need for additional conversion.

Also fixes #2285.

[alexeyshockov]

The build will be green when #2278 is merged. Other tests are OK.

[alexeyshockov]

Although this PR is self-contained, guzzle/psr7#269 makes IDN support overall better.

[sagikazarmark]

@alexeyshockov Thanks! Can you please rebase your branch?

[alexeyshockov]

@sagikazarmark, done

[atymic]

Great to see support for these coming.

[alexeyshockov]

@sagikazarmark, could you take a look also at this for the next release?

[sagikazarmark]

Yeah, it's on my list, thanks. I recall a related PR in the psr7 package. Is it a dependency of this PR? Do we have to raise the minimum version?

[alexeyshockov]

Is it a dependency of this PR?

No, the PR to PSR7 is not a dependency, but an addition (very convenient, though, because without it Guzzle will struggle with locales, that's why hardcoded it to 'C' in the tests).

Do we have to raise the minimum version?

Minimum version of PHP stays the same (IDN support is enabled only when intl extension is available).

[sagikazarmark]

Minimum version of PHP stays the same (IDN support is enabled only when intl extension is available).

I meant PSR7 package version, but it's not necessary then.

[alexeyshockov]

Yeah, I mixed things with PSR7 package a bit, will prepare a separate PR to 1.x branch

[Nyholm]

I think this looks good. Could you add a test where you use base_uri?

[Nyholm]

If you rebase this on master and then add a test where you use an internationalized domain in base_uri, then I would be happy to merge.

[alexeyshockov]

@Nyholm, done

[Nyholm]

Thank you