ENH: do not convert mixed-integer type indexes to datetimeindex
closes #3877.

@jreback any thoughts/opinions here? i'm not sure if i really want this because of `eval` or because it makes sense, feel free to tell me that i'm too gung-ho :) on the one hand just because datetimeindexes have join precedence over integers they shouldn't then assume that the joined index of integers refer to dates. on the other hand how often (or not) does one have an object index containing mixed integers and datetimes?
 hmm....maybe this should be changed a bit to only allow certain types of indicies on a time-series join.

e.g. string, integer (only if time-series convertible)

I would raise I think if inferred_type != object or the TypeError is triggered

of course see what this breaks
 one problem with the second point is that i believe all integers except for reserved ones like `nan`/`inf` are time-series convertible. i think mixed-integer datetimeindexes are too ambiguous to just assume that the ints are datetimes thus only this change...i will play around to see if being more strict with joins breaks the entirety of pandas
 sweet only breaks 4 tests 2 of which are trying to convert, easy enough to change, need to look at others
 @jreback legacy support is broken if you raise and only allow strings to be converted. currently it's really only an issue with converting mixed-integer type indexes. unless one of the goals is to raise whenever an object index is being used...thus discouraging users from using it. my change here preserves legacy support and fixes the issue of conversion i can do the raise if u want but is it too soon for htat?
 that's fine then
I always want to have general solutions rather that specific related to types but
sometimes unavoidable
 i agree that it's annoying to have to special case this but the int<->datetime mapping makes it necessary when u already have an object index :(
 @jreback anything else u think i should add here? if not should i merge?
 go ahead
 @jreback u were right after all, all these issues are related to this ....
 the fix or the symtom?
 symptom
