closes JuliaData/Missings.jl/issues/61

Added handling of missing/nothing inputs for parse and tryparse. Returns nothings to stay consistent with parse returns during invalid parses.

Author: pepelovesvim

base/parse.jl

@@ -380,6 +380,8 @@ parse(::Type{T}, s::AbstractString; kwargs...) where T<:Real =
     convert(T, tryparse_internal(T, s, true; kwargs...))
 parse(::Type{T}, s::AbstractString) where T<:Complex =
     convert(T, tryparse_internal(T, s, firstindex(s), lastindex(s), true))
+parse(::Type{T}, s::Union{Missing, Nothing}) where T<:Number = nothing
 
 tryparse(T::Type{Complex{S}}, s::AbstractString) where S<:Real =
     tryparse_internal(T, s, firstindex(s), lastindex(s), false)
+tryparse(::Type{T}, s::Union{Missing, Nothing}) where T<:Number = nothing