Fix constructor spacing

Tabs shouldn't be used for alignment in the middle of a line. In different code-viewing environments, such as when sharing with others, different values of tab-widths make the code appear unaligned.

Both the [WordPress Coding Standards](http://make.wordpress.org/core/handbook/coding-standards/php/#indentation), and the wider [PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md#24-indenting) standards (among others) suggest that spaces should be used for mid-line alignment.