aw_interpolate fails when `tid` and `sid` are the same

[chris-prener]

aw_interpolate fails when `tid` and `sid` are the same

[chris-prener]

The multipolygon branch now contains a fix for this issue as well. It also includes a unit test that confirms the fix.

[chris-prener]

The fix for this looked like so:

> si_zcta2 <- rename(si_zcta, id = GEOID)
> aw_interpolate(
+   si_zcta2,
+   tid = id,
+   source = si_tract,
+   sid = GEOID,
+   weight = "sum",
+   output = "tibble",
+   extensive = "popE"
+ )
# A tibble: 13 x 2
   id      popE
 * <chr>  <dbl>
 1 10301 41705.
 2 10302 16504.
 3 10303 25834.
 4 10304 41365.
 5 10305 44373.
 6 10306 53132.
 7 10307 12564.
 8 10308 27879.
 9 10309 31901.
10 10310 22839.
11 10311   464.
12 10312 66662.
13 10314 88100.