Allow priority value of 0

[bakerkretzmar]

This PR ensures that a URL's priority can be set to 0.

The <priority> tag accepts values from 0.0 to 1.0, but this package currently strips out values like 0 and 0.0 (it's possible to work around this by passing something like 0.01, which is rounded down to 0.0 and kept, but that's not ideal).

Omitting the <priority> tag is not the same as setting it to 0, because the default priority is 0.5.

Docs: https://www.sitemaps.org/protocol.html#xmlTagDefinitions

[freekmurze]

Thanks!