BUG: Update merge to support rasterio 1.2+

[snowman2]

• Related to Update to rasterio 1.2 features #180, Allow merge to open one dataset at a time rasterio/rasterio#1994
• Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

Currently broken with 1.2.

[codecov]

Codecov Report

Merging #217 (e4fcdd8) into master (60c4552) will decrease coverage by 0.10%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
- Coverage   93.24%   93.14%   -0.11%     
==========================================
  Files          11       11              
  Lines        1244     1254      +10     
==========================================
+ Hits         1160     1168       +8     
- Misses         84       86       +2     

Impacted Files	Coverage Δ
rioxarray/merge.py	95.94% <80.00%> (-2.50%)	⬇️

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