Fixed support for HasOne relationships when sorting

[kevinwheeler]

In the original code, line 128 is as follows:
$ownerKey = $relationship->getOwnerKeyName();

However, HasOne relationship instances don't have a getOwnerKeyName method.

[kevinwheeler]

I have only thought through the BelongsTo and HasOne cases. Those were the only cases included in the original code before I modified it anyways. Perhaps my changes also fix BelongsToMany relationships, but I haven't looked into it.

Adding support for HasMany relationships may be as simple as changing
if ($relationship instanceof HasOne) {
on line 131 to
`if ($relationship instanceof HasOne || $relationship instanceof HasMany ) {

and importing HasMany.

I'm unlikely to spend time thinking through if this would work, but we could always add this line of code anyways with a comment that caveats that it may not work. Let me know if you would like me to do this.