Debug merge for label uniqueness checking

Author: gdalle

docs/src/tutorial_graphs.md

@@ -64,10 +64,10 @@ julia> has_vertex(cities, 4)
 false
 ```
 
-Note that we can't add the same city twice:
+Note that we can't add the same city (i.e. vertex label) twice:
 
 ```jldoctest graphs
-julia> cities[:London] = "United Kingdom"
+julia> add_vertex!(cities, :London, "Italy")
 false
 
 julia> nv(cities)

src/graphs.jl

@@ -80,7 +80,7 @@ Add a vertex to MetaGraph `g` with label `label` having metadata `data`.
 
 Return true if the vertex has been added, false incase the label already exists or vertex was not added.
 """
-function Graphs.add_vertex!(g::MetaGraph, label, val)
+function Graphs.add_vertex!(g::MetaGraph, label, data)
     if haskey(g,label)
         return false
     end