Different code- vs. label- API for `has_edge` vs. `add_edge!`

Example below:

g = SimpleGraph()
mg = MetaGraph(g, String, Nothing, Nothing)
for c in 'a':'d'
    add_vertex!(mg, string(c))
end
add_edge!(mg, "a", "b")
has_edge(mg, "a", "b")
ERROR: method has_edge not implemented.

I.e., add_edge! works on the labels of the MetaGraph but has_edge works with the codes. This is especially surprising if you (contrary to recommendations) go with Integer labels:

g = SimpleGraph()
mg = MetaGraph(g, Int, Nothing, Nothing) # <--- Int label
for i in 3:6
    add_vertex!(mg, i)
end
add_edge!(mg, 3, 4)
has_edge(mg, 3, 4)

(where, current API requires us to do has_edge(mg, code_for(mg, 3), code_for(mg, 4)) to get a meaningful result.

Start by reproducing the two Julia examples in the issue and compare the has_edge and add_edge! entry points, including their handling of labels and codes. A resolution should make the API behavior consistent for labeled vertices and cover both the string-label and integer-label cases.