azure-storage-common/src/Common/Internal/RestProxy.php
protected function addOptionalHeader(array &$headers, $key, $value)
    {
        Validate::isArray($headers, 'headers');
        Validate::canCastAsString($key, 'key');
        Validate::canCastAsString($value, 'value');

        if (!is_null($value) && Resources::EMPTY_STRING !== $value) {
            $headers[$key] = $value;
        }
    }
protected function addOptionalHeader(array &$headers, $key, $value)
    {
        Validate::isArray($headers, 'headers');
        Validate::canCastAsString($key, 'key');
        Validate::canCastAsString($value, 'value');

        if (!is_null($value) && Resources::EMPTY_STRING !== $value) {
            $headers[$key] = $value;
        }
    }
Adds optional header.

Doesn't add the value if it satisfies empty().

@param array  &$headers The HTTP header parameters.
@param string $key      The HTTP header name.
@param string $value    The HTTP header value.

@return void