Rewind the keyFileStream before each read

[mcrumm]

When using either keyFile or keyFileStream to provide credentials to the ServiceBuilder, if the user's credentials expire between requests, an error will occur when trying to load credentials from the file stream, because the pointer is at the end of the stream.

Here's the output from the test prior to calling $keyFileStream->rewind() within the ServiceBuilder:

PHPUnit 5.1.3 by Sebastian Bergmann and contributors.

.....................EE.......................................... 65 / 96 ( 67%)
...............................                                   96 / 96 (100%)

Time: 250 ms, Memory: 16.77Mb

There were 2 errors:

1) Google\Cloud\Tests\RequestWrapperTest::testCredentialsFromKeyFileStreamCanBeReadMultipleTimes with data set #0 (array(Closure Object (...), Closure Object (...), '{"type":"authorized_user","cl..."abc"}'))
array_key_exists() expects parameter 2 to be array, null given

/Users/mike/Code/mcrumm/gcloud-php/vendor/google/auth/src/CredentialsLoader.php:118
/Users/mike/Code/mcrumm/gcloud-php/src/RequestWrapper.php:177
/Users/mike/Code/mcrumm/gcloud-php/tests/RequestWrapperTest.php:115

2) Google\Cloud\Tests\RequestWrapperTest::testCredentialsFromKeyFileStreamCanBeReadMultipleTimes with data set #1 (array(Closure Object (...), Closure Object (...), '/Users/mike/Code/mcrumm/gclou...e.json'))
array_key_exists() expects parameter 2 to be array, null given

/Users/mike/Code/mcrumm/gcloud-php/vendor/google/auth/src/CredentialsLoader.php:118
/Users/mike/Code/mcrumm/gcloud-php/src/RequestWrapper.php:177
/Users/mike/Code/mcrumm/gcloud-php/tests/RequestWrapperTest.php:115

FAILURES!
Tests: 96, Assertions: 115, Errors: 2.

[dwsupplee]

Thanks for this @mcrumm, Looks good to me.