ToLowercaseUnderscore should not separate digits in property names.

This addresses two related issues. 

First is that Should_not_put_double_underscore fails on a property name that contains digits. Property names "like_this_0" turn into names "like_this__0".

Second is that digits should not be split by underscores.

I found this when debugging ServiceStack.OrmLite.PostgreSQL.PostgreSqlNamingStrategy, a DTO with class Alias("click_trends_60m") was looking for "click_trends__6_0_m".