fix setindex updates to existing elements of NT

[magerton]

Fixes setindex so that function can "update" arguments in place and added test. Looks like was an issue with the order of the arguments in merge function. Seems to relate to #40

MWE shows current issue

using NamedTuples
x = @NT(a = 1)
@show setindex(x, :a, 2) # (a=1)
@show merge(@NT(a=2), x) # (a=2) CORRECT!
@show merge(x, @NT(a=2)) # (a=1)

[codecov-io]

Codecov Report

Merging #71 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   73.18%   73.18%           
=======================================
  Files           1        1           
  Lines         179      179           
=======================================
  Hits          131      131           
  Misses         48       48

Impacted Files	Coverage Δ
src/NamedTuples.jl	73.18% <100%> (ø)	⬆️

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