Functions to load user by name or email should be case insensitive

The functions user_load_by_name() and user_load_by_mail() are just wrappers for entity_load() ultimately. They use the deprecated parameter $conditions to pass the account name and the email address respectively. However, these conditions are added using a simple equality comparison. Thus, differences in the case are not ignored. This behavior differs from the user register form, where the client wouldn't be able to create another account using an username or an e-mail address already registered by just changing their case.

Furthermore, this behavior leads to errors on contrib projects. For example, I use the LDAP module, which relies on these functions to check if an account with the provided username or email exists. Hence, multiple accounts can be created by simply changing words case. I don't think it's a problem with LDAP module as the aforementioned functions do not work as expected.