Use appropriate precision for ang2PixRing

[ziotom78]

• Add test case for https://github.com/ziotom78/Healpix.jl/issues/93
• Fix issue #93

[codecov[bot]]

Codecov Report

Merging #94 (e5d7960) into master (8f48dec) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #94   +/-   ##
=======================================
  Coverage   87.43%   87.43%           
=======================================
  Files          19       19           
  Lines        1154     1154           
=======================================
  Hits         1009     1009           
  Misses        145      145           

Impacted Files	Coverage Δ
src/pixelfunc.jl	89.74% <100.00%> (ø)

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