index entries of same method name are missing in index-all.html

Some method names are used between different classes: for example, abs(), asBoolean(). But they are showed only once in index-all.html. So there are smaller number of index entries than actual existences.

It's caused by wrong implementation of comparator of TreeSet in DocGenerator.groovy. I sent a pull request to fix it. Please check it out.

https://github.com/groovy/groovy-core/pull/384