Avoid double done call in join and add tests

Some iterators might mutate their state in the `done()` method (eg Task)
and that will give wrong results when `join` calls `done` twice per
iteration. This fixes that, and add a few tests to `join` that were
missing.

Fixes #9178

base/string.jl

@@ -1394,19 +1394,23 @@ function print_joined(io, strings, delim, last)
     end
     str, i = next(strings,i)
     print(io, str)
-    while !done(strings,i)
+    is_done = done(strings,i)
+    while !is_done
         str, i = next(strings,i)
-        print(io, done(strings,i) ? last : delim)
+        is_done = done(strings,i)
+        print(io, is_done ? last : delim)
         print(io, str)
     end
 end
 
 function print_joined(io, strings, delim)
     i = start(strings)
-    while !done(strings,i)
+    is_done = done(strings,i)
+    while !is_done
         str, i = next(strings,i)
+        is_done = done(strings,i)
         print(io, str)
-        if !done(strings,i)
+        if !is_done
             print(io, delim)
         end
     end

test/strings.jl

@@ -1249,3 +1249,20 @@ end
 # This caused JuliaLang/JSON.jl#82
 @test first('\x00':'\x7f') === '\x00'
 @test last('\x00':'\x7f') === '\x7f'
+
+# Tests of join()
+@test join([]) == ""
+@test join(["a"],"?") == "a"
+@test join("HELLO",'-') == "H-E-L-L-O"
+@test join(1:5, ", ", " and ") == "1, 2, 3, 4 and 5"
+@test join(["apples", "bananas", "pineapples"], ", ", " and ") == "apples, bananas and pineapples"
+
+# issue #9178 `join` calls `done()` twice on the iterables
+type i9178
+    nnext::Int64
+    ndone::Int64
+end
+Base.start(jt::i9178) = (jt.nnext=0 ; jt.ndone=0 ; 0)
+Base.done(jt::i9178, n) = (jt.ndone += 1 ; n > 3)
+Base.next(jt::i9178, n) = (jt.nnext += 1 ; ("$(jt.nnext),$(jt.ndone)", n+1))
+@test join(i9178(0,0), ";") == "1,1;2,2;3,3;4,4"