Fix clear cache for delete db

[MarkFull]

The current way of clearing cache when deleting db does not work well.
The reason is include does not override the delete! method.

In this way this PR is fixing the problem: reopen the class, alias_method delete! to another name, clear cache in the redefined method, to achieve the overridden effect.

[ktaragorn]

I work with @MarkFull On further investigation we found that the couchrest_model way of monkeypatching delete! by defining the method in a different module and then including was not taking effect. So potentially that is why this PR was needed, and also why this PR is probably broken. If the delete! is monkey patched by reopening the class instead, then it should work.

[ktaragorn]

Looks like recreate! calls delete! so this change shouldnt be needed. From some debugging looks like the new version of delete! here doesnt seem to take hold.

[ktaragorn]

The problem with include is that methods of a class cannot be overridden by modules included in that class 

http://stackoverflow.com/a/5944385/1520364

[ktaragorn]

This version of the changes were tested to work with recreate! as well.

[samlown]

@MarkFull @kanterov thanks for looking at this! There are a couple of points however. It would be great to have a test demonstrating that this is not currently working as expected, and secondly, given that we no longer support Ruby < 2.0, we can safely use the Module#prepend method. I'll see if I can add these quickly.

[samlown]

Confirmed and fixed in this commit: 38bce62 Thanks!