Added asertion to check folder isn't NULL in plainTextBodyRenderingOperationWithMessage and htmlBodyRenderingOperationWithMessage
Added assert to ensure folder name is not NULL when doing plainText/HTML body rendering
Merge branch 'master' of https://github.com/KieranMcGrady/mailcore2
Remove assertions
Merge pull request #1014 from KieranMcGrady/master

Added assertion to check folder isn't NULL
Updated yandex provider