SPARK-6480 [CORE] histogram() bucket function is wrong in some simple edge cases

Fix fastBucketFunction for histogram() to handle edge conditions more correctly. Add a test, and fix existing one accordingly

Author: Sean Owen <sowen@cloudera.com>

Closes #5148 from srowen/SPARK-6480 and squashes the following commits:

974a0a0 [Sean Owen] Additional test of huge ranges, and a few more comments (and comment fixes)
23ec01e [Sean Owen] Fix fastBucketFunction for histogram() to handle edge conditions more correctly. Add a test, and fix existing one accordingly