Correctly call API when searching by source URL

[psrpinto]

Fixes #172

The API was been called incorrectly. I also took the liberty of URL-encoding the field, otherwise it could break the URL parsing, for example if sourceurl would be http://foo?bar=baz, because this would result in the following URL:

Note there are two ?

/wp-json/index.php?rest_route=/try-wp/v1/subjects/blog-post&sourceurl=http://foo?bar=baz

[psrpinto]

Thanks for finding the issue @ashfame!

[ashfame]

Thanks for catching that we were not encoding the url!