Fix 2 bugs in readRev:

[flimzy]

• Composite values before _rev were parsed token-by-token, rather than discarding the entire value
• A value of "_rev" would have confused it

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.02% ⚠️

Comparison is base (102b906) 82.50% compared to head (79af993) 82.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #728      +/-   ##
==========================================
- Coverage   82.50%   82.49%   -0.02%     
==========================================
  Files          85       85              
  Lines        9009     9012       +3     
==========================================
+ Hits         7433     7434       +1     
- Misses       1368     1369       +1     
- Partials      208      209       +1     

Files Changed	Coverage Δ
couchdb/chttp/chttp.go	96.35% <33.33%> (-0.70%)	⬇️

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