GetWordBoundaryChars would return "allcaps" for "ALL_CAPS" instead of "ac". This
would manifest as ranking (for instance) "STDIN_FILENO" ahead of "stdin" for
query "std", which is terrible.
This bug has been present in YCM for many months, but no one noticed the issue.
Fixes#272.
The point is that we want to prefer candidates that have the query characters
"earlier" in their text, e.g. "xxabcxxx" over "xxxxxabc" for "abc" query.