Use dims kw arg for dropdims

dropdims is not a simple renaming of squeeze, but instead expects dims as a
keyword argument.

src/Compat.jl

@@ -2006,7 +2006,7 @@ end
 
 # https://github.com/JuliaLang/julia/pull/28303
 if VERSION < v"0.7.0-beta2.143"
-    const dropdims = squeeze
+    dropdims(X; dims = throw(UndefKeywordError("cat: keyword argument dims not assigned"))) = squeeze(X, dims)
 end
 
 include("deprecated.jl")

test/runtests.jl

@@ -1932,5 +1932,11 @@ end
 
 # 0.7.0-beta2.143
 @test isdefined(Compat, :dropdims)
+if VERSION < v"0.7.0-beta2.143"
+    let a = reshape(Vector(1:4),(2,2,1,1)), b = reshape(Vector(1:4), (2,2,1))
+        @test Compat.dropdims(a; dims=3) == b
+        @test_throws UndefKeywordError Compat.dropdims(a)
+    end
+end
 
 nothing