ensure that getstring resets the position of IO

[KristofferC]

Before this PR, this happened:

julia> source = IOBuffer("\"str1\" \"str2\"");

julia> opt = Parsers.Options(; quoted=true);

julia> res = Parsers.xparse(String, source, 1, 0, opt);
Parsers.Result{PosLen}(5, 7, PosLen(0x0000000000200004))

julia> @test Parsers.getstring(source, res.val, opt.e) == "str1"
Test Passed

julia> res = Parsers.xparse(String, source, 1 + res.tlen, 0, opt)
Parsers.Result{PosLen}(5, 3, PosLen(0x0000000000900001))

julia> @test Parsers.getstring(source, res.val, opt.e) == "str2"
Test Failed at REPL[7]:1
  Expression: Parsers.getstring(source, res.val, opt.e) == "str2"
   Evaluated: "s" == "str2"
ERROR: There was an error during testing

[quinnj]

Good catch. The source as IO codepath may have occasional bugs like this as it's not as widely used as the AbstractVector{UInt8} paths.