[MessageTrait] Fix withAddedHeader() with array $value

[siwinski]

This pull request fixes an issue when withAddedHeader()'s input param $value is an array. After the changes in this pull request, output will match Guzzle's and Diactoros' output.

Issue example:

<?php

$guzzleResponse = (new GuzzleHttp\Psr7\Response('200', array('test' => 'one')))
    ->withAddedheader('test', array('two'));
echo "Guzzle header: ";
print_r($guzzleResponse->getHeader('test'));

$diactorosResponse = (new Zend\Diactoros\Response('php://memory', 200, array('test' => 'one')))
    ->withAddedheader('test', array('two'));
echo "Diactoros header: ";
print_r($guzzleResponse->getHeader('test'));

$ringCentralResponse = (new RingCentral\Psr7\Response('200', array('test' => 'one')))
    ->withAddedheader('test', array('two'));
echo "RingCentral header: ";
print_r($ringCentralResponse->getHeader('test'));

Issue example output:

Guzzle header: Array
(
    [0] => one
    [1] => two
)
Diactoros header: Array
(
    [0] => one
    [1] => two
)
RingCentral header: Array
(
    [0] => one
    [1] => Array
        (
            [0] => two
        )

)

[siwinski]

Failed Travis build should be fixed by #5

[clue]

Good catch, thank you for looking into this! 👍

[kirill-konshin]

Pls rebase and re run the tests

[siwinski]

Pls rebase and re run the tests

@kirill-konshin done

[kirill-konshin]

Thanks for the PR!