fix buffer interfaces

[bicycle1885]

Supercedes #7.

[codecov-io]

Codecov Report

Merging #46 into master will decrease coverage by 0.48%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   95.24%   94.76%   -0.49%     
==========================================
  Files           7        6       -1     
  Lines         715      668      -47     
==========================================
- Hits          681      633      -48     
- Misses         34       35       +1

Impacted Files	Coverage Δ
src/EzXML.jl	4.54% <ø> (-95.46%)	:arrow_down:
src/node.jl	98.38% <100%> (+2.86%)	:arrow_up:
src/xpath.jl	94.59% <0%> (-0.53%)	:arrow_down:
src/error.jl	96.15% <0%> (-0.28%)	:arrow_down:
src/document.jl	99.12% <0%> (+2.42%)	:arrow_up:
src/streamreader.jl	95.87% <0%> (+3.07%)	:arrow_up:

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