reinstate "useless" String() inner constructor (#16212)

Author: StefanKarpinski

base/boot.jl

@@ -220,6 +220,8 @@ abstract DirectIndexString <: AbstractString
 
 immutable String <: AbstractString
     data::Array{UInt8,1}
+    # required to make String("foo") work (#15120):
+    String(d::Array{UInt8,1}) = new(d)
 end
 
 include(fname::String) = ccall(:jl_load_, Any, (Any,), fname)

test/strings/basic.jl

@@ -1,5 +1,9 @@
 # This file is a part of Julia. License is MIT: http://julialang.org/license
 
+# constructors
+@test String([0x61,0x62,0x63,0x21]) == "abc!"
+@test String("abc!") == "abc!"
+
 # {starts,ends}with
 @test startswith("abcd", 'a')
 @test startswith("abcd", "a")