fix whereparams that are unions

[oxinabox]

I think I need this for
invenia/Nabla.jl#189

Before it would get this wrong:

julia> foo(x::T) where T<:Union{Float32,Bool} = 2x
foo (generic function with 1 method)


julia> signature(@which foo(true))
Dict{Symbol,Any} with 3 entries:
  :name        => :foo
  :args        => Expr[:(x::T)]
  :whereparams => Any[:(T <: var"Union{Bool, Float32}")]

julia> splitdef(:(foo(x::T) where T<:Union{Float32,Bool} = 2x))
Dict{Symbol,Any} with 5 entries:
  :args        => Any[:(x::T)]
  :body        => quote…
  :name        => :foo
  :head        => :(=)
  :whereparams => Any[:(T <: Union{Float32, Bool})]

This fixes it so that signature matches splitdef

[codecov]

Codecov Report

Merging #11 into master will increase coverage by 0.70%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #11      +/-   ##
==========================================
+ Coverage   98.60%   99.31%   +0.70%     
==========================================
  Files           4        4              
  Lines         143      145       +2     
==========================================
+ Hits          141      144       +3     
+ Misses          2        1       -1     

Impacted Files	Coverage Δ
src/method.jl	100.00% <100.00%> (+1.69%)	⬆️
src/function.jl	98.73% <0.00%> (-0.04%)	⬇️

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