BrowserUpdateTransformer can get into invalid state - must override recycle()

If a form throws an exception during transformation (see https://issues.apache.org/jira/browse/COCOON-1785) then the BrowserUpdateTransformer gets into an invalid state, and futher request by any user or session that happens to use the invalid transformer receive the entire form document inside of the bu:document tag. The client side ajax javascript is then unable to process the resulting update.

The attached patch (license granted to ASF) overrides recycle() and fixes this problem.