Fix Overzoom issues with tile layers

[tvblomberg]

This pull request should fix issues with calculating zoom for maxNativeZoom and calculating tilesize.
Right now the maxNativeZoom option does not do anything for tileLayers. This worked correctly in version 2.1.5. Currently users will get white tiles when overzooming. This is my first pull request so please let me know if I am doing something wrong. This should fix issue #1250

Here is the example of the two versions:
2.1.5: https://jsfiddle.net/jfwkq0s4/4/
3.1.1: http://jsfiddle.net/dbpfcoqo/6/

[olaf-cloudgenix]

I hit this too when setting detectRetina to true on the tileLayer. It tries to fetch either tiles for the completely wrong geography, or tiles that don't exist (resulting in 404 errors). This pull request fixes it.

[riastrad]

I'd like to take this along with the next minor release, but will be using #1323 to track adding it against latest master instead of wrestling with a +2 year rebase.