REGR: allow merging on object boolean columns
Fixes https://github.com/pandas-dev/pandas/issues/21119

This is a quick/hacky fix for the specific case in that issue, i.e. merging boolean / object boolean columns. 

However, I have the feeling we should do a more general solution for cases where object dtyped columns might be actually mergeable. The problem is that inferring the type can be costly? (and eg when having actual strings object columns, this inferring was also unnecessary and thus will unneeded slow down `merge`?)