Fix eltype for chained generators

[iamed2]

Fixes #90

Still need to add a test

[codecov-io]

Codecov Report

Merging #97 into master will increase coverage by 0.29%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   94.11%   94.41%   +0.29%     
==========================================
  Files           1        1              
  Lines         374      376       +2     
==========================================
+ Hits          352      355       +3     
+ Misses         22       21       -1

Impacted Files	Coverage Δ
src/Iterators.jl	94.41% <50%> (+0.29%)	:white_check_mark:

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[iamed2]

If we want to continue to support 0.4 I'll make an additional test file for 0.5+ tests that use generator syntax.