MONGOID-5221/5222: Fix mongoization + demongoization of non-empty string as Integer/Float

[johnnyshields]

This is a partial fix for MONGOID-5222 and MONGOID-5221

[p-mongo]

The ticket is for demongoization, this PR changes mongoization and demongoization. The to_i/to_f calls should remain as they were, they are not equivalent to calling Integer()/Float().

[johnnyshields]

@p-mongo I've clarified in the ticket the behavior should be identical for both mongoization and demongoization. There's no reason to convert the string "Foobar" to 0; it should be nil as it is non-sensical.

Note that :demongoize is aliased to :mongoize for these classes, and this is further reason to keep it consistent.

(BigDecimal by the way already does the correct thing both mongoization and demongoization)

[johnnyshields]

@p-mongo please consider merging this at this time. While I appreciate there is an epic to fully review mongoization/demongoization, this PR is a "quick win" for a Float/Integer which are very commonly used.

[p-mongo]

I responded in https://jira.mongodb.org/browse/MONGOID-5222.