fix inexacterrors caused by type coercion in broadcast

[tlnagy]

The following plot fails on master, but is in our documentation:

julia> using Gadfly

julia> plot(x=rand(500), y=rand(1:4, 500), Stat.y_jitter(range=0.5), Geom.point)
Error showing value of type Gadfly.Plot:
ERROR: InexactError()
 in macro expansion at ./broadcast.jl:129 [inlined]
 in macro expansion at ./simdloop.jl:73 [inlined]
 in macro expansion at ./broadcast.jl:123 [inlined]
 in _broadcast!(::Base.#+, ::Array{Int64,1}, ::Tuple{Tuple{Bool},Tuple{Bool}}, ::Tuple{Tuple{Int64},Tuple{Int64}}, ::Tuple{Array{Int64,1},Array{Float64,1}}, ::Type{Val{2}}) at ./broadcast.jl:117
 in broadcast!(::Function, ::Array{Int64,1}, ::Array{Int64,1}, ::Array{Float64,1}) at ./broadcast.jl:172
 in apply_statistic(::Gadfly.Stat.JitterStatistic, ::Dict{Symbol,Gadfly.ScaleElement}, ::Gadfly.Coord.Cartesian, ::Gadfly.Aesthetics) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/statistics.jl:1752
 in apply_statistics(::Array{Gadfly.StatisticElement,1}, ::Dict{Symbol,Gadfly.ScaleElement}, ::Gadfly.Coord.Cartesian, ::Gadfly.Aesthetics) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/statistics.jl:38
 in render_prepare(::Gadfly.Plot) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/Gadfly.jl:740
 in render(::Gadfly.Plot) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/Gadfly.jl:799
 in display(::Base.REPL.REPLDisplay{Base.REPL.LineEditREPL}, ::MIME{Symbol("text/html")}, ::Gadfly.Plot) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/Gadfly.jl:1062
 in macro expansion at ./multimedia.jl:143 [inlined]
 in display(::Gadfly.Plot) at /Users/tamasnagy/.julia/v0.5/Gadfly/src/Gadfly.jl:1014
 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_interface(::Base.Terminals.TTYTerminal, ::Base.LineEdit.ModalInterface) at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?
 in run_frontend(::Base.REPL.LineEditREPL, ::Base.REPL.REPLBackendRef) at ./REPL.jl:903
 in run_repl(::Base.REPL.LineEditREPL, ::Base.##930#931) at ./REPL.jl:188
 in _start() at ./client.jl:360
 in _start() at /Applications/Julia-0.5.app/Contents/Resources/julia/lib/julia/sys.dylib:?

https://dcjones.github.io/Gadfly.jl/stat_y_jitter.html

This PR fixes this problem. Working towards #879.

[coveralls]

Coverage increased (+0.007%) to 65.61% when pulling 7c3590d on tlnagy:fix-broadcast-type-coercion into f6183ac on dcjones:master.

[coveralls]

Coverage increased (+0.007%) to 65.61% when pulling 7c3590d on tlnagy:fix-broadcast-type-coercion into f6183ac on dcjones:master.

[tlnagy]

Thanks @shashi ! 🎉