add examples and update tests

base/random.jl

@@ -1479,6 +1479,18 @@ from `chars` if specified, and of upper- and lower-case letters and
 the digits 0-9 otherwise. The optional `rng` argument specifies a
 random number generator, see [Random Numbers](@ref).
 
+# Examples
+```julia-repl
+julia> randstring()
+"c03rgKi1"
+
+julia> randstring(MersenneTwister(0), 'a':'z', 6)
+"wijzek"
+
+julia> randstring("gcat")
+"tgtcaatc"
+```
+
 !!! note
     The collection `chars` can be any collection of characters
     (of type `Char` or `UInt8`)

test/random.jl

@@ -539,15 +539,21 @@ let g = Base.Random.GLOBAL_RNG,
 end
 
 # test randstring API
-let b = ['0':'9';'A':'Z';'a':'z'],
-    c = 'a':'z'
+let b = ['0':'9';'A':'Z';'a':'z']
     for rng = [[], [MersenneTwister(0)]]
         @test length(randstring(rng...)) == 8
         @test length(randstring(rng..., 20)) == 20
         @test issubset(randstring(rng...), b)
-        @test issubset(randstring(rng..., c), c)
-        s = randstring(rng..., c, 20)
-        @test length(s) == 20
-        @test issubset(s, c)
+        for c = ['a':'z', "qwèrtï", Set(Vector{UInt8}("gcat"))],
+            len = [8, 20]
+            s = len == 8 ? randstring(rng..., c) : randstring(rng..., c, len)
+            @test length(s) == len
+            if eltype(c) == Char
+                @test issubset(s, c)
+            else # UInt8
+                @test issubset(s, map(Char, c))
+            end
+        end
     end
+    @test randstring(MersenneTwister(0)) == randstring(MersenneTwister(0), b)
 end