Addressing zero-byte reference when using namespace

[davissp14]

Honor the namespace when using zero byte. This will intercept any zero-byte references and rewrite them to target everything under the current namespace..

Addressing: #136

[codecov]

Codecov Report

Merging #135 (59a237f) into master (a69267f) will decrease coverage by 0.03%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   99.17%   99.14%   -0.04%     
==========================================
  Files          39       39              
  Lines        2072     2096      +24     
==========================================
+ Hits         2055     2078      +23     
- Misses         17       18       +1     

Impacted Files	Coverage Δ
lib/etcdv3/namespace/kv/requests.rb	95.83% <83.33%> (-4.17%)	⬇️
spec/etcdv3_spec.rb	100.00% <100.00%> (ø)

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