Only 2d index is required to first in compound index

Fix for https://github.com/fakemongo/fongo/issues/65
Not sure I understand your question about including a check for the key equaling "loc" since the field name is user-defined.

However, I realized that my change broke the creation of 2dsphere indexes. Committed fix and also created a couple tests (didn't look hard enough for 2d tests for my previous commit).