do_merge_xts() unnecessarily coerces to REAL when 'x' and 'y' aren't the same type and one isn't returned

[joshuaulrich]

do_merge_xts() shouldn't coerce x or y to REAL if they're not the same type and one isn't returned.

lgl <- xts(rep(TRUE, 10), .Date(1:10))
int <- x + 1
# should return logical, but returns double
dbl <- merge(lgl, int, retside = c(TRUE, FALSE))

packageVersion('xts')
## [1] '0.14.0'