FIX: enable stream context to pass through CWP egres proxy, fixes #156

[smindel]

Replacing fsockopen() with file_get_contents() enables you to set a default stream context accounting for the CWP egres proxy like this:

use SilverStripe\Core\Environment;

if(Environment::getEnv('SS_OUTBOUND_PROXY') && Environment::getEnv('SS_OUTBOUND_PROXY_PORT')) {
    stream_context_set_default([
        'http' => [
            'proxy' => sprintf(
                'tcp://%s:%s',
                Environment::getEnv('SS_OUTBOUND_PROXY'),
                Environment::getEnv('SS_OUTBOUND_PROXY_PORT')
            ),
            'request_fulluri' => true,
        ]
    ]);
}

The above code in app/_config.php sets up the egres proxy not only for this module but all outgoing connections that respect default stream contexts in your entire project.

[codecov-io]

Codecov Report

Merging #157 into master will increase coverage by 4.3%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master     #157     +/-   ##
===========================================
+ Coverage     73.14%   77.45%   +4.3%     
+ Complexity      126      124      -2     
===========================================
  Files             4        4             
  Lines           324      306     -18     
===========================================
  Hits            237      237             
+ Misses           87       69     -18

Impacted Files	Coverage Δ	Complexity Δ
src/GoogleSitemap.php	81.29% <0%> (+9.32%)	58 <0> (-2)	⬇️

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