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

[chris-prener]

See Issue #6 for a reprex - since @mfherman is using two census geometries, the ID is GEOID in both cases and we get this for output:

> aw_interpolate(
+   si_zcta,
+   tid = GEOID,
+   source = si_tract,
+   sid = GEOID,
+   weight = "sum",
+   output = "tibble",
+   extensive = "popE"
+ )
# A tibble: 13 x 2
   GEOID  popE
 * <chr> <dbl>
 1 10301    NA
 2 10302    NA
 3 10303    NA
 4 10304    NA
 5 10305    NA
 6 10306    NA
 7 10307    NA
 8 10308    NA
 9 10309    NA
10 10310    NA
11 10311    NA
12 10312    NA
13 10314    NA

Renaming one of the ID variables from GEOID to anything else fixes it. Not sure how to handle this yet. For now, @mfherman, rename your target ID variable until we address this.

[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.

[chris-prener]

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