Adds Server Health Check

[montymxb]

Adds ParseClient::getServerHealth as a quick way to check on the server health without having to make an arbitrary request to a functional endpoint, and without needing keys.

$health = ParseClient::getServerHealth();
if($health['status'] === 200) {
    // all good!

} else if(isset($health['error']) {
   // error & message instead

   $code = $health['error'];
   $message = $health['error_message'];
   // handle the error...

}

Any response is decoded if possible and added under 'response'.

$health = ParseClient::getServerHealth();

// get status message from the server
$response = $health['response'];
$serverStatus = $response['status'];
// ok

'response' may be a string if it was unable to be decoded. Any additional information the server provides will be available under 'response'.

Tests are added, README is updated and a few adjustments were made in ParseClient to avoid having to maintain duplicate code.

[codecov]

Codecov Report

Merging #366 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
+ Coverage   98.92%   98.93%   +<.01%     
==========================================
  Files          36       36              
  Lines        3360     3389      +29     
==========================================
+ Hits         3324     3353      +29     
  Misses         36       36

Impacted Files	Coverage Δ
src/Parse/ParseClient.php	99.27% <100%> (+0.08%)	⬆️

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