match_results operator[] no longer accepts enums

[ymett]

Pull request 208 Suppress conversion warnings in match_results broke the ability to pass an unscoped enum to match_results operator[] (and related functions).

This ability was particularly useful because it allowed giving expressive names to matches without having to number them individually (which is error prone).

E.g.:

enum { banana = 1, apple, orange };
boost::regex r("(banana)|(apple)|(orange)");

allowing
match[apple]

with the index value updating automatically when changing the regex (so long as the enum is updated to match).

Two possible fixes:

1. Quick: Add an explicit test for enums
2. Correct: Allow anything which converts implicitly to int, restoring earlier behaviour