`disj` of the last element of the set results in non-iterable collection

To reproduce:

using FunctionalCollections

s = pset(1,2,3)
# => PersistentSet{Int64}(2, 3, 1)
disj(s, 1)

will output:

PersistentSet{Int64}(Error showing value of type FunctionalCollections.PersistentSet{Int64}:
ERROR: MethodError: no method matching next(::FunctionalCollections.SparseNode{FunctionalCollections.PersistentArrayMap{Int64,Void}}, ::Array{Any,1})
Closest candidates are:
  next(::SimpleVector, ::Any) at essentials.jl:171
  next(::Base.MethodList, ::Any) at reflection.jl:260
  next(::IntSet, ::Any) at intset.jl:187
  ...
 in next(::FunctionalCollections.PersistentHashMap{Int64,Void}, ::Tuple{Array{Pair{Int64,Void},1},Array{Any,1}}) at C:\Users\alexe_000\.julia\v0.5\FunctionalCollections\src\PersistentMap.jl:173
 in next(::FunctionalCollections.PersistentHashMap{Int64,Void}, ::Tuple{Array{Pair{Int64,Void},1},Array{Int64,1}}) at C:\Users\alexe_000\.julia\v0.5\FunctionalCollections\src\PersistentMap.jl:174
 in next at .\generator.jl:25 [inlined]
 in collect_to!(::Array{Int64,1}, ::Base.Generator{FunctionalCollections.PersistentHashMap{Int64,Void},FunctionalCollections.##30#31}, ::Int64, ::Tuple{Array{Pair{Int64,Void},1},Array{Int64,1}}) at .\array.jl:340
 in collect_to_with_first!(::Array{Int64,1}, ::Int64, ::Base.Generator{FunctionalCollections.PersistentHashMap{Int64,Void},FunctionalCollections.##30#31}, ::Tuple{Array{Pair{Int64,Void},1},Array{Int64,1}}) at .\array.jl:327
 in collect(::Base.Generator{FunctionalCollections.PersistentHashMap{Int64,Void},FunctionalCollections.##30#31}) at .\array.jl:308
 in show(::IOContext{Base.Terminals.TTYTerminal}, ::FunctionalCollections.PersistentSet{Int64}) at C:\Users\alexe_000\.julia\v0.5\FunctionalCollections\src\PersistentSet.jl:81
 in display(::Base.REPL.REPLDisplay{Base.REPL.LineEditREPL}, ::MIME{Symbol("text/plain")}, ::FunctionalCollections.PersistentSet{Int64}) at .\REPL.jl:132
 in display(::Base.REPL.REPLDisplay{Base.REPL.LineEditREPL}, ::FunctionalCollections.PersistentSet{Int64}) at .\REPL.jl:135
 in display(::FunctionalCollections.PersistentSet{Int64}) at .\multimedia.jl:143
 in print_response(::Base.Terminals.TTYTerminal, ::Any, ::Void, ::Bool, ::Bool, ::Void) at .\REPL.jl:154
 in print_response(::Base.REPL.LineEditREPL, ::Any, ::Void, ::Bool, ::Bool) at .\REPL.jl:139
 in (::Base.REPL.##22#23{Bool,Base.REPL.##33#42{Base.REPL.LineEditREPL,Base.REPL.REPLHistoryProvider},Base.REPL.LineEditREPL,Base.LineEdit.Prompt})(::Base.LineEdit.MIState, ::Base.AbstractIOBuffer{Array{UInt8,1}}, ::Bool) at .\REPL.jl:652
 in run_interface(::Base.Terminals.TTYTerminal, ::Base.LineEdit.ModalInterface) at .\LineEdit.jl:1579
 in run_frontend(::Base.REPL.LineEditREPL, ::Base.REPL.REPLBackendRef) at .\REPL.jl:903
 in run_repl(::Base.REPL.LineEditREPL, ::Base.##932#933) at .\REPL.jl:188
 in _start() at .\client.jl:360

So, the error happens only on output or when you try to collect or filter or other way iterate the collection. Taking length, on the other hand works.

Tested on: Windows 10, Julia 0.5, FunctionalCollections (master)

Reproduce the issue with disj(pset(1,2,3), 1), then inspect PersistentSet.jl around its show method and PersistentMap.jl around next at lines 173–174. Verify the resulting set can be displayed, collected, and filtered after removing the final element, while length still works.