Docs integration should not throw errors

```
julia> using PyCall, PyPlot

julia> x= 1:10
1:10

julia> y= 1:10
1:10

julia> fig, ax = subplots()
(Figure(PyObject ), PyObject )

julia> ax[:plot](x,y)
1-element Array{PyObject,1}:
PyObject

help?> ax
search: ax axis axes axvspan axvline axhspan axhline Axes3D max maximum maximum! maxintfloat delaxes minmax typemax findmax

ERROR: PyError ($(Expr(:escape, :(ccall(#= /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/conversions.jl:87 =# @pysym(PyString_AsStringAndSize), Cint, (PyPtr, Ptr{Ptr{UInt8}}, Ptr{Int}), po, _ps_ptr, _ps_len)))))
TypeError('expected bytes, NoneType found')

Stacktrace:
[1] pyerr_check at /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/exception.jl:60 [inlined]
[2] pyerr_check at /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/exception.jl:64 [inlined]
[3] macro expansion at /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/exception.jl:84 [inlined]
[4] convert(::Type{String}, ::PyObject) at /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/conversions.jl:87
[5] getdoc at /home/pfitzseb/.julia/packages/PyCall/0jMpb/src/PyCall.jl:857 [inlined]
[6] getdoc(::Any, ::Any) at ./docs/Docs.jl:254
[7] doc(::Base.Docs.Binding, ::Type) at /home/pfitzseb/Documents/Git/julia/usr/share/julia/stdlib/v1.2/REPL/src/docview.jl:86
[8] doc(::Base.Docs.Binding) at /home/pfitzseb/Documents/Git/julia/usr/share/julia/stdlib/v1.2/REPL/src/docview.jl:85
[9] top-level scope at /home/pfitzseb/Documents/Git/julia/usr/share/julia/stdlib/v1.2/REPL/src/docview.jl:285
```

Originally reported in https://github.com/JunoLab/Juno.jl/issues/217. Seems to happen for all Julia versions between 0.7 and master, afaict.

PyCall と PyPlot の REPL セッションを再現し、その後 PyCall/src/conversions.jl:87、PyCall.jl:857、Julia's Docs.jl:254、REPL/docview.jl:86 を調べます。プロットされたオブジェクトのヘルプを要求しても、報告された TypeError が発生しなくなれば作業完了です。

索引モデルが issue の本文から書いたものです。