`d[k] = v`: replace key when new values are assigned in dicts.

Setup:

	julia> d = Dict()
	Dict{Any,Any}()

	julia> d[1] = "foo"
	"foo"

	julia> d
	{1=>"foo"}

	julia> d[1.0] = "bar"
	"bar"

Before:

	julia> d
	{1=>"bar"}

After:

	julia> d
	{1.0=>"bar"}

The new behavior seems far less surprising to me.