Raise TypeError for PostgreSQL ranges with alphabet

Passing valid Ruby Alphabetic ranges such as
"a..z" into pg range columns currently fails with
a syntax error coming from the db:
So when saving `"a".."z"` in a `int4range` PostgreSQL column it raises:
```
<ActiveRecord::StatementInvalid>
"PG::Error: ERROR:  invalid input syntax for integer ...
```

 it should probably throw a `TypeError: can't cast Range to int4range`.

test case : https://gist.github.com/gzohari/6096239